Leatherarse is the term given in BDSM
to a submissive, slave or bottom
who has developed such a resistance to beatings, whippings and spankings that their arse has become leatherlike and will no longer bruise or welt in the manner that would be expected of an average person subjected to similar torture.
Leatherarse can apply to skin on the back, thighs, breasts or anywhere else one can be beaten not just specifically the buttocks.
Some submissives have natural leatherarse but most who attain it develop it over time and/or through training.
Leatherarse, although sometime annoying, is often used as a positive term and seen as something a submissive and their Dom/me can be proud of.
A submissive with leatherarse is by definition almost always also a pain slut
Domme: That submissive of yours can really take a beating; you've been caning him for an hour and he's only just rosey!
Master: Yup, he's a total leatherarse.
Submissive: Can I see the bruises from the spanking you had yesterday?
Slave: No, I've nothing to show because I've got leatherarse.