A twirly is what it is called when the cock is "twirled" in a circular motion, traditionally with the index finger. It can either be self-administered, or performed by an intimate partner. A classic rendition is the reach-around twirly. This happens when, of two homosexual males, the giver of ass-sex, reaches around, and twirls the receiver's cock.
"Yo, Tommy, what are you up to tonight?"
"I'll probably just sit in front of the computer and twirl it to A Night in Paris for a couple of hours." (heterosexual).
"Oh, yesss, ass-sexxxx."
"You want a twirly with that?"
"Shit yeah I do!"
"Good, 'cause I've been working out my indexes."
"Well, then twirl away motherfucker!" (homosexual).