Merde

Means "shit" (literally or interjectional or figuratively).

Means also "good luck" when said to someone who will accomplish a performance or pass an exam.

Merde ! Fais attention où tu marches ! Il y a de la merde partout dans ce parc ! Shit! Pay attention where you walk! There is shit everywhere in this parc!
Ce travail, quelle merde ! This work is really shit!
Tu vas à ton examen de physique ? Bonne merde ! Are you going to your physics exam? Good luck!