What is the meaning of Mierda?

Shit.

shit

shit (expression of worry, failure)

shit (solid excretory product evacuated from the bowel)

shit (solid excretory product evacuated from the bowels; feces.)

shit

piece of shit; a bad thing; an object of poor quality.

drunkenness

shit! (expression of worry, failure, shock, only for negative situations)

Source: wiktionary.org