A "butt kiss" is when you take a shit and it piles up and/or leans over to make contact with your ass.
That turd was so long, it actually gave me a butt kiss when it snapped off!
The touching of one smokers lit cigarette to another's unlit cigarette with both cigs still in their respective owner's mouths. Used to light up in the absence of a lighter.
Dude 1: Got a light?
Dude 2: Nah, we can butt kiss though.
Dude 1: Gay, but do want a smoke.