How to say it: You should already know how it's pronounced, but just in case: "mayrd."
What it means: General equivalent of "shit," usually as in "aw shit."
When it's used: Does not commonly refer to waste matter (that's caca). Usually uttered slowly in exasperation, as when having just dropped one's keys down the hole of a Turkish toilet: meeeerd-euh.
Raunchy relatives: emmerder, as in Tu m'emmerdes!—"You're bugging the shit out of me." (Literally, "You be-shit me.")
tas de merde—pile of shit.