“Je vous dis merde” or “Merde” (“shit” in English) is a theatrical saying meaning “good luck.” Superstitious thespians believe that it’s bad luck to wish someone good luck. “Merde” has been cited in France since at least 1940 and was popularly used in Western Union “good luck” telegrams during World War II. “Merde” has been used by ballet dancers of the New York City Ballet (see the 1977 and 1978 citations, below).
 
Another opposite saying for “good luck” used in the theater is “Break a leg!”
 
   
Wikipedia: Break a leg
“Break a leg” is a well-known idiom in theatre which means “good luck.” It is typically said to actors and musicians before they go on stage to perform. The origin of the phrase is obscure.
 
The expression reflects a theatrical superstition in which wishing a person “good luck” is considered bad luck. The expression is sometimes used outside the theatre as superstitions and customs travel through other professions and then into common use. Among professional dancers, the phrase “break a leg” is replaced with “merde”.
 
(Oxford English Dictionary)
merde, int. and n.
Etymology:  < French

merde merd n.; in use as interjection after similar use in French (late 12th cent. in Old French). In sense A. 2 after similar use in French theatrical slang.
(...)
Chiefly Theatre slang. ‘Good luck!’ (said to a performer before a performance).
1961 I. Jefferies It wasn’t Me! x. 132   ‘Oh, well, Merde!’ He stuck out his hand and I shook it. ‘Merde to you.’
1985 J. Trapido et al. Internat. Dict. Theatre Lang. 536   Merde, The dancer’s equivalent‥of the actor’s ‘Break a leg’ or ‘Have a good show’.
1990 M. Caine Coward’s Chrons. (BNC) 88   Two of the musicians, carrying the usual cans of Gold Label, pop their heads in and shout: ‘Break a leg, gang.’ ‘Merde,’ we chorus back, feeling terribly ‘thespian’.
     
Google Books
If I Laugh:
The chronicle of my strange adventures in the great Paris exodus—June 1940
By Rupert Downing
London: G. G. Harrap
1941
Pg. 59:
He waved his hand and said, “Je vous dis merde.”
 
The phrase is an exceedingly popular one among certain classes in France, and, on its face value, extremely rude, but there exists a superstition that it is unlucky to wish people good luck. If the superstition is right, then the obvious thing to do (as the fates are contrary) is to wish them something very different, to be positively insulting, in fact. “Je vous dis merde” is what Noah would have said to his dove when he pushed it out of the porthole — if Noah had been a Frenchman.
 
Google Books
Moondrop to Gascony
By Anne Marie Walters
London: Macmillan
1947
Pg. 9:
”Merde . . .” said everyone as we walked away. No one was supposed to say “good luck”; it brought bad luck. “Merde” was the only wish of good fortune allowed.
     
,a href=“https://www.google.com/books/edition/Footloose_in_France/2rcVirS8XhcC?hl=en&gbpv=1&bsq=merde+%22good+luck%22&dq=merde+%22good+luck%22&printsec=frontcover”>Google Books
Footloose in France
By Horace Sutton
New York, NY: Rinehart
1948
Pg. 87:
But nobody ever says “Good luck!” In the superstitious creative set, to say good luck means bad luck, so everybody offers best wishes by just saying merde.
 
Google Books
Ten Days in August; a novel
By Bernard Frizell
New York, NY: Simon and Schuster
1956
Pg. 83:
“Aren’t you going to tell us merde for good luck?” Mignot asked.
”Merde,” said Pierrot.
“Thanks.”
 
Google Books
“But he doesn’t know the territory.”
By Meredith Willson
New York, NY: Putnam
1957
Pg. 188:
“Show people are pretty superstitious, you know. You’re only supposed to say ‘merde’ and things like that, I think. ‘Break your leg’ and things like that.”
   
Google Books
Your Career in the Theater
By Bruce Savan
Garden City, NY: Doubleday
1961
Pg. 58:
This is one of the oldest ways of wishing good luck in the theater and is known all over the world. “Hals und Bein brechen” means “Break your neck” and is used the same way. Backstage, “Beaucoup de merde” or simply “Merde” means luck.
 
Google Books
Watchman, what of the night?
By Jed Harris
Garden City, NY: Doubleday
1963
Pg. 12:
For it would certainly lose its special cachet among theatrical people, who customarily telegraph their friends “Merde” for good luck on opening nights.
 
(This practice began during the last war, when Western Union conceived it to be its patriotic duty to refuse to transmit messages of good wishes. But telegrams bearing the magic word “Merde” were promptly and cheerfully delivered.)
       
Google Books
September Child:
The story of Jean Dalrymple
By Jean Dalrymple
New York, NY: Dodd, Mead
1963
Pg. 57:
I soon found out it is “bad luck” to wish “good luck” in the theater, so you say something like, “Break your neck,” or if you are French or English or consider yourself a sophisticate, you say, “Merde.”
 
Google Books
Once More from the Beginning
By Robert Merrill with Sandford Dody
New York, NY: Macmillan
1965
Pg. 10:
“Break a leg, Mr. Merrill.”
”Merde, amico.”
”Bocca lupo.”
“Good luck, Bob.”
 
Google Books
The Playmakers
By Stuart W. Little and Arthur Cantor
New York, NY: W. W. Norton & Company, Inc.
1970
Pg. 90:
Implicit in the relationship of actor and audience are violence and hostility: the cliche phrases associated with this relationship abound in references to destruction, death, and sexual conquest.  “Merde” and “Break a leg” are common actor’s telegrams.
       
Newspapers.com
15 December 1974, Oakland (CA) Tribune, ‘Hunger Is a Ballet Dancer’s Best Friend” by Paul Hertelendy, pg. 27-ENT, col. 6:
Performance time drew near, with the usual good-luck exchanges: “Break a leg” (traditional), “Merde” (from the French), or the Russian “T-t-t-t-t,” where you pretend to spit over the other’s should for good luck.
 
Newspapers.com
22 March 1975, The Citizen (Ottawa, ON), “The interview: Price Mireille Mathieu must pay for success,” pg. 73, col. 3:
The questions have run out, and I wish her good luck with her performance.
 
“No, no,” she replies. “In French we say ‘merde’”.
   
Newspapers.com
8 June 1976, The Age (Melbourne, Victoria, Australia), “Using French slang is fun” by Manuel Gelman, pg. 15, col. 2:
The very common merde! presents a unique case. In its literal meaning of “excrement” it must be handled, figuratively, with kid gloves. but as expletive, expressing admiration, annoyance or indignation, I have heard it used by Cabinet Ministers, ambassadors and highly respectable ladies without embarrassment or offence. The tone means everything. To say “merde” is a standard way of wishing friends good luck before an exam or before going on stage at a premiere. The word has no English twin, and has none of the ugly crudity of its four-letter Anglo Saxon equivalent.
 
Google Books
Ballet as a Body Language:
The anatomy of ballet for student and dance lover
By Joan MacConnell and Teena MacConnell
New York, NY: Harper & Row
1977  
Pg. ?:
Theatrical etiquette requires one always to say the opposite of what one wants to say. Therefore, “Break a leg” means “Have a great performance!”
 
In the New York City Ballet, the most commonly used phrase for these occasions is the French five-letter word merde.
 
Google Books
Giselle Considers Her Future
By Evan Zimroth
Columbus, OH: Ohio State University Press
1978
Pg. 26:
Merde, says everyone in the wings, merde, merde, breakaleg. Unfortunately, the dancer quite likely will break a leg since the lining of her costume is loaded down with totem objects supposed to promote good luck,
 
Google Books
Ned and Jack: a drama
By Sheldon Rosen
New York, NY: Samuel French
1982
Pg. 32:
“I hope this message will reach you in time to bring you my best wishes ever for much, much success, and in the proper tradition of the theatre— break a leg. Merde.”
 
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
(...)
Merde
Theatre performers opt for “break a leg,” but dancers commonly wish each other “merde,” which directly translates to “shit” in French. The origin of this tradition traces back to 19th-century Paris when attendees of the Paris Opera Ballet would pull up to the famed Palais Garnier in horse-drawn carriages. The more audience members the more carriages, the more horses, the more… merde.