It’s a term in BDSM that that may sounds weird. However, it exists. Although some psychologists thinks that this is a deviation, some people enjoy this. As an exhibitionist enjoys his or her nudity in front of public.
Pitslave is a slave (in BDSM consensual circumstance) with an armpit fetish.
It means that she or he loves her or his armpits to be naked, exposed, vulnerable. A pitslave likes when her or his wrists chained pulled above the head. And also the kiss of the whip, the teasing caress of a cruel fingertip, a hot or electric needle, a tongue tip licking or soft lips suckling, clips torturing her or him on the armpit for hours, making it so sensitive. Those make a pitslave feels so naked.
She or he will feel naked when the side of her or his arm touches the chest’s side so intimately. Feels the the armpits getting hotter and watery, then trickling down. That’s when a pitslave feel vulnerable, helpless and humiliated.
A pitslave has a fetish issue and submission tendency. So, it’s part of her or his slavery and what defines being a (consensual) slave for the person. By exposing her or his armpits, she or he could turn her or him on.