A man, properly a merchant. A chap-man is a merchantman or tradesman. “If you want to buy, I'm your chap.” A good chap-man or chap became in time a good fellow. Hence, A good sort of chap, A clever chap, etc. (Anglo-Saxon, ceap-mann.)
An awkward customer is an analogous phrase.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894