E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
A man, properly a merchant. A chap-man is a merchantman or tradesman. If you want to buy, Im 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.)