the phrase comes from the fact that ugly girls have to go an extra mile and do things out of the ordinary for guys to have intrest in them
Devin: "Yea, I think she'll lick your butt though"
Peter: "What makes you say that?"
Devin: "well, I was talking to Julia, who's friends with Alexis, and the way she talks about her made me feel like she was trying to drop hints about her being ugly"
Jake: "I got a new girlfriend"
Mike: "Will she lick your butt?"
Mainly for use in bars/clubs.