Someone who is being forcefully manipulated in their asshole while yelling for there mammy "mommy" hence the term mammy rammer.
Mike: Hey Josh! What are doin.
Josh: Ah nothin just sittin here hangin out.
Mike: Hey did you hear what happened to Paul last night?!
Josh: No, whats up!?
Mike: I was walkin by this dark alley last night and I saw Paul bent over a dumpster handcuffed with a big black guy sticking a dildo up Paul's ass and Paul was screaming for his mommy.
Josh: What?!Man did he get the Mammy Rammer!
Mike: Yeah what a pussy!