When someone refers to "Bill Cozbying" someone, it means doing them in the ass and then having them suck you off. In other words, it's ass to mouth. Defined most notably by the song "Bill Cozby" by One Cool Guy.
Lyrics from "Bill Cozby" by One Cool Guy:
"You’re gonna wanna bend down and give me some
Turn around then give me a hum
As you take all of my root
In the poop chute"
A woman giving a blow job raps her lips around her teeth to prevent biting the male recipients penis. They begin to resemble bill cosby.
She gives great Bill Cozbys