“Break a leg!” is a theatrical saying meaning—somewhat oddly—“Good luck!” A joke was posted on Twitter by Ralph Tetta on July 5, 2011:
 
“In showbiz, they tell you to ‘break a leg,’ which is why every show has a cast. #jokesforsale #cheap”
 
“Hear a joke, why do people say ‘break a leg’ before every show? Because every play has a cast lolol” was posted on Twitter on August 4, 2012.
 
         
Twitter
Ralph Tetta‏
@RalphTetta
In showbiz, they tell you to “break a leg,” which is why every show has a cast. #jokesforsale #cheap
6:26 PM - 5 Jul 2011
 
Twitter
danielle‏
@xdaniellelopez
Replying to @NiallOfficial
@NiallOfficial Hear a joke, why do people say “break a leg” before every show? Because every play has a cast lolol
3:49 PM - 4 Aug 2012
     
23 February 2016, Daily Mail (London, UK), “On This Day” by James Black, pg. 20:
JOKE OF THE DAY WHY do we tell actors to ‘break a leg’? Because they all like to be in a good cast.
 
3 September 2016, Fraser Coast Chronicle (Maryborough, Queensland), pg. 14:
WHY do we tell actors to “break a leg”?
Because every play has a cast!
   
Twitter
Meade Drama‏
@Meadewayoffbway
Why do people say “break a leg” before you go onstage? Because every play has a cast. #justjokes #meadewayoffbway
7:39 AM - 21 Feb 2017
     
Odyssey
74 One-Liners That Will Make You Laugh Your Head Off
Kelli Frye
Jul 24, 2017
At Chattanooga, Tennessee
(...)
36. Why do people say “break a leg” before going on stage? BECAUSE EVERY PLAY HAS A CAST!
 
Twitter
lorraine Jo King‏
@LahLahBean
#tuesdaytitters Why do people say “Break a leg” before you go on stage.. Because every play has a cast #jokesoftheday
6:27 AM - 25 Jul 2017
   
Reddit—Cleanjokes
Why do we tell actors to “break a leg?” (self.cleanjokes)
submitted February 5, 2018 by madazzahatter
Because every play has a cast.