"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...
Which, of course, while it is a diret word-for-word translation, has perfectly antinomic meaning in each language.