"Je te dis merde" before a performance would translate to "break a leg".
The supersition goes that you should not reply with "merci" ("thanks") but "je prends" ("I'll take it ?")
There are unverifiable explanations for why "merde" is used by actors ; my favorite one is that it is related to horse carriage carrying theatre-viewers to the theatres ; you're wishing actors that a lot of horse will carry a lot of viewers, and do the other thing...
The supersition goes that you should not reply with "merci" ("thanks") but "je prends" ("I'll take it ?")
There are unverifiable explanations for why "merde" is used by actors ; my favorite one is that it is related to horse carriage carrying theatre-viewers to the theatres ; you're wishing actors that a lot of horse will carry a lot of viewers, and do the other thing...