Green’s Dictionary of Slang

chap v.2

[? SE chap, to cause (the skin) to crack]

to be unsatisfactory, to be bad; note mis-defined as an adj. in cit.

Online Sl. Dict. 🌐 chap adj 1. inadequate or displeasing. (‘That test chapped.’).

In phrases

chap someone’s ass/rear (v.)

see under ass n.