The act of sticking a finger up someone's
bum during sex, either with their express request, reluctant permission, or (more amusingly) completely out of the blue.
Applies to both gay and straight.
Consider it a sort of polite alternative, or a beginner's version if you would, of fisting.
It comes from the acronym
FUB - Finger Up Bum.
Derivatives
like 'to
fub', 'fubbed', 'well I'll be fubbed', 'fubstep' and so on are known to exist and are applicable according to context.
"I was having sex with this 8/10
girl yesterday and out of the blue, she starts fubbing me. No warning whatsoever! At first I didn't know what to do, but she was
pretty fit so I just rode it out... Fubbed and proud!"