Last night that bitch I was with almost knocked herself out cold when she had her head too close to my balls as Cocktimus Prime was transforming. Dumb bitch, so I used Cocktimus Prime's main weapon, the baseball bat swing, and knocked her skank ass out anyway. All hail Cocktimus Prime.
It means: a mutant combination between cockroach and mouse.
Robin: So what was it, a cockroach or a mouse?
Lily: It was a cockamouse!
Robin: What?
Lily: It's some sort of mutant combination of the two.
It's as if a cockroach and a mouse, you know...