It’s often considered, in the theater and elsewhere, to be bad luck to wish someone “good luck.” Instead, some people say, “Toi, toi, toi,” and spit three times. “Toi” is from the German “Teuful” (the devil). The expression probably came into the theater from German opera stars. 
 
“Here he raises his eyes to heaven, raps on a barrel—touching wood for luck—and spits three times, ‘toi, toi, toi,’ against the evil eye” was printed in Town & Country (New York, NY) in December 1938. “The slim German girl who played the heroine spit three times—toit! toi! toi!—on my table collar, the Continental actors’ way of wishing good luck” was printed in Town & Country (New York, NY)  in August 1939. ” The proper way to combat evil influences hanging around gremlin-like, is to say ‘Toi, toi, toi’” was printed in the column “Lights of New York” by L. L. Stevenson in the Houma (LA) Times on July 31, 1942.
   
“Break a leg!” and “Merde!” are other sayings for good luck.
 
       
Wiktionary: toi, toi, toi
Etymology
From German toi, toi, toi, which see for more.
Interjection
toi, toi, toi
1. (theater, opera) A superstitious expression of encouragement prior to a performance.
Synonyms: (theatre) break a leg, (considered to bring misfortune if used in the theatre) good luck
Usage notes
This expression may not be readily understood outside the context of the opera, theatre or music.
 
Wikipedia: Toi, toi, toi
“Toi toi toi” (English: /ˈtɔɪ ˈtɔɪ ˈtɔɪ/) is an expression used in the performing arts to wish an artist success in an imminent performance. It is similar to “break a leg” and reflects a superstition that wishing someone “good luck” is in fact bad luck.
 
Origin
There are many theories as to the origin of Toi toi toi as an idiom. In folklore it was used to ward off a spell or hex, often accompanied by knocking on wood or spitting. One origin theory sees “toi toi toi” as the onomatopoeic rendition of spitting three times, a common practice in many parts of the world to ward off evil spirits. Saliva traditionally had demon-banishing powers. Another theory claims the origin to be a threefold warning of the devil (Teufel, pronounced as TOY-fell) in German (n.b. not the French “toi”).
 
Also from Rotwelsch tof and from Yiddish tov (“good”, derived from the Hebrew טוב and with phonetic similarities to the Old German tiuvel “Devil”).
   
December 1938, Town & Country (New York, NY), “Vintage Rhine” by Elan von Mumm Thornton, pg. 84, col. 2:
“Well, gnadige Frau, it might be a great wine year, if—” and here he raises his eyes to heaven, raps on a barrel—touching wood for luck—and spits three times, “toi, toi, toi,” against the evil eye.
 
Google Books
August 1939, Town & Country (New York, NY), “Dear General—what a surprise!” by Ludwig Bemelmans, pg. 61, col. 3:
EVERYBODY wished every one the good luck. The slim German girl who played the heroine spit three times—toit! toi! toi!—on my table collar, the Continental actors’ way of wishing good luck. She also said, “Hals und Beinbruch!”—a phrase that skiers and acrobats shout to each other.
 
Newspapers.com
31 July 1942, Houma (LA) Times, “Lights of New York” by L. L. Stevenson, pg. 4, col. 5:
Old Belief: According to Rise Stevens, whose husband, Walter Szurovy, is of Hungarian birth, the European equivalent of wishing good luck to a performer about to go on stage is: “I hope you sing like a pig.” It’s an old, old belief. Miss Stevens continued, that to wish a performer good luck is certain to bring bad luck. The proper way to combat evil influences hanging around gremlin-like, is to say “Toi, toi, toi.” The words, in insignia, are represented by three bars. Mr. Szurovy being stationed in an army camp in California cannot say “Toi, toi, toi” to his wife before her performances. So her designed a ring for her, three diamond bars set in pink gold, which she wears as a reminder that, though he may be 3,000 miles away, he is plugging for her.
     
Google Books
The World of Bemelmans:
An Omnibus
By Ludwig Bemelmans
New York, NY: Viking Press
1955
Pg. 175:
Everybody wished everyone else good luck. The slim German girl who played the heroine spat three times—toi, toi, toi!—on my sable collar, the Continental actors’ way of wishing good luck. She also said, “Hals und Beinbruch!”
 
Newspapers.com
14 November 1955, Stockton (CA) Record, “Met Opera Opens Season Tonight” by H. D. Quigg, pg. 7, col. 2:
NEW YORK, Nov. 14 (UP)—Tonight at 7:58, with the knights and ladies of America society gathered in glittering and august assembly for the headline social event of the year, Richard Tucker’s good wife will look him in the face and say quickly: “Toi-toi-toi.”
 
Then she will spit three times.
(...)
TRADITIONAL WISH
Mrs. Tucker will “toi-toi-toi” in his face and spit 3 times over his shoulder as they stand in the wings. That’s the traditional good-luck wish given a singer just before he goes on the stage.
 
That is, it’s the tradition if he’s singing in France, or in the French language. In Italian-Language opera, the well-wisher says “in bocco a lupo” (in the mouth of the wolf) for good luck. In German it’s “halsundbeingbruck” (may you break your back and your bones).
 
In Austria, it’s no words at all—just a gesture—three kicks in the rear. This not only gives the singer spirit but helps propel him on stage, if he’s headed in the right direction.
             
Google Books
Your Career in the Theater
By Bruce Savan
Garden City, NY: Doubleday
1961
Pg. 58:
“Toi, toi, toi”—always spoken three times—is a good luck expression used by Italians. In ballet, one of the most popular carriers of good luck is a kick in the seat of the pants.
 
Google Books
A Star in the Family:
An Autobiography in Diary Form
By James McCracken and ‎Sandra Warfield
New York, NY: Coward McCann & Geoghegan
1971
Pg. 29:
Backstage in the different theaters of the world there are many strange good luck customs . ... an explosive toi, toi, toi which is a refined vocalization of the spit (I prefer it) or “Hals und Beinbruch” -which means “break your leg.”
 
Google Books
Supernatural on Stage:
Ghosts and Superstitions of the Theatre
By Richard Huggett
New York, NY: Taplinger Publishing Company
1975
Pg. 109:
Good- luck wishes in the operatic world are more repulsive and less hygienic than in the theatre. In Germany they spit three times, or they recite the sound which spitting is traditionally supposed to make which is toi-toi-toi.
 
Googel Books
The Language of Theatre
By Martin Harrison
New York, NY: Routledge
1998
Pg. 284:
toi toi toi Used mainly in European theatre , though it has gained limited currency in Britain, toi toi toi is an explosive phrase emitted in circumstances when the staider English actor might mutter ‘break a leg’: a way of wishing a performer good luck before they go on stage. It is a stylised and santised version of spitting over one’s shoulder, and another example of superstition in the theatre.
   
Google Books
Ritual, Performance, Media
Edited by Felicia Hughes-Freeland
London, UK: Routledge
1998
Pg. 112:
There are special good luck expressions such as ‘merde!’ or ‘toi-toi-toi!’, which must not be answered back (especially not close to or on the stage), since that is believed to bring bad luck. So it is to say thank you for ‘toi-toi’ cards or presents, usually small dolls, soft toy animals or sweets that may refer to the ballet production in one way or another.
 
NPR
For A Bit Of Luck, Opera Singers Say A Melange O’ Tois
August 11, 2014 4:14 PM ET
Heard on All Things Considered
MELISSA BLOCK, HOST:
 
Time for some more trade lingo. We’ve asked you to send us phrases or bits of slang from your world - insider terms of the trade that would stump people on the outside. And today we have this phrase.
 
MICHELLE HACKMAN: Toi, toi, toi.
 
BLOCK: Toi, toi, toi - spelled T O I. That was sent to us by listener Michelle Hackman of Cary, Illinois. She’s a professional opera singer and she told us she heard toi, toi, toi for the first time when she was in college. She was backstage before a performance.
 
HACKMAN: A voice teacher said it to one of the performers. And I said what is that? I’ve never heard that before in my life. It basically means break a leg. That’s the term we use instead of break a leg or instead of good luck.

BLOCK: And when you heard it and you asked about it, what did they tell you?

HACKMAN: They said it was just the way to say good luck. And there was also a spitting motion. I’ll pretend to spit over the shoulder. It’s to ward off evil spells or bad luck. It’s also - comes from the German term for devil, which is Teufel. And so they say it could be a shortened version of that. So speaking the devil’s name to ward him off.
       
Playbill
32 Theatre Terms Everyone Should Know
From “break a leg” to “strike,” here’s what they mean and where they came from.
BY RUTHIE FIERBERG
AUGUST 10, 2019
(...)
Toi, Toi, Toi … In Bocca al Lupo … Chookas
Around the world, there are yet more phrases to substitute for well wishes. “Toi, toi, toi” in Germany emanates from the German/Yiddish history of spitting to ward off evil spirits and bad luck. “In bocca al lupo” means “in the wolf’s mouth” and the correct response is “crepi il lupo,” which means “may the wolf die”—warding off a bad omen. Australians say “chookas,” which is believed to be a permutation of “chook” or chicken. In the old days, chicken was considered a delicacy; by saying “chookas” you are hoping the performance will go well and make money so that the performer can afford a gourmet meal.
 
London Theatre
The stories behind theatre’s well-known phrases and sayings
Will Longman
25 May, 2022, 10:51
(...)
“Toi, Toi, Toi”
If you’re involved in a production, fellow cast members may say “Toi, Toi, Toi” which could sound like nonsense. Actually, it’s another phrase to wish good luck to other actors. It derives from German/Yiddish tradition, with people spitting “Toi” to rid off any evil spirits and in turn, bad luck.
 
Twitter
Wagtail
@PlischkeThomas
Replying to @jacecore
Toi, toi, toi - as they say in Germany before somebody gets on stage to wish them luck and chase evil spirits away. 😊
8:51 AM · Nov 12, 2022·Twitter Web App
 
Twitter
Portobello Express
@Portob_Express
Replying to @ChrisBodMusic
As we say in Austria, toi toi toi and good luck❤
6:10 AM · Nov 28, 2022·Twitter for Android