A.T.B. stands for Anal Tongue Bath. Its the art of licking your partners arsehole for sexual stimulation or to clean their brown balloon knot. It is not the most plesant activity and can lead to bad breath depending on the diet of the partner
Man. "Oi honey, im up for a good ride tonight"
Woman. "No problem, you can get me going by giving me an A.T.B."
Man. "Fuck you, we had an indian take away last night, your ring will be nasty"
Woman. "Fine, have a wank instead!"