An abbr. of motherfucking in an effort to be more politically correct, "PG", or safe for work.
That m'er'f'in bitch was a fucking whore ass dipshit at the party last night.
Mom! I'm on my m'er'f'in way back home. Chill!
Awwww yeahhhhh, m'er'f'in cheeze burgerzzz...