It is the censored way to say motherfuckerthat was derived from the warping of the word motherfuckerin various rap songs. It can be used as an indirect way to curse if one doesn't feel comfortable saying motherfucker and it also adds a bit of charm to the act of cursing. It can also be spelled in the slang way, othamufcka. It can also be used in the present progressive form as othermufcking or othamufckin'.
-"My teacher gave me a D on my project."
-"Man, that bitch is a straight up othamufcka!"
-"I'm tired of seein' your othamufckin' face!"
-"Dude, he, like, is a total douche bag. He's a freaking cunt othermufcker bro!"