Chapman is an English surname derived from the
Old English occupational name
céapmann “marketman, monger, merchant”, from the verb
céapan, cypan “to buy or sell” and the noun form
ceap "barter, business; a purchase." Alternate spellings include Caepmon, Cepeman, Chepmon, Cypman(n), and Shapman. (By 1600, the occupational name
chapman had come to be applied to an itinerant dealer in particular, but it remained in use for both "customer, buyer" and "merchant" in the 17th and 18th centuries. Modern chiefly British slang
chap “man" arose from the use of the abbreviated word to mean a customer, one with whom to bargain.)