cheapyoudaoicibaDictYouDict[cheap 词源字典]
cheap: [16] The adjectival use of cheap in English is quite recent, but the word itself goes back a long way. Its ultimate source is the Latin noun caupō ‘tradesman’, which was borrowed into Germanic in prehistoric times. Among its descendants were German kaufen ‘buy’, Old English cēapian ‘trade’ (the possible source of chop, as in ‘chop and change’), and the Old English noun cēap ‘trade’.

In Middle English times this came to be used in such phrases as good chepe, meaning ‘good bargain’, and by the 16th century an adjectival sense ‘inexpensive’ had developed. The original sense ‘trade’ is preserved in the personal name Chapman, which until the 19th century was an ordinary noun meaning ‘trader’ (it is the source of chap ‘fellow’).
=> chap, chop[cheap etymology, cheap origin, 英语词源]
cheap (adj.)youdaoicibaDictYouDict
"low in price, that may be bought at small cost," c. 1500, ultimately from Old English noun ceap "traffic, a purchase," from ceapian (v.) "trade," probably from an early Germanic borrowing from Latin caupo "petty tradesman, huckster" (see chapman).

The sense evolution is from the noun meaning "a barter, a purchase" to "a purchase as rated by the buyer," hence adjectival meaning "inexpensive," the main modern sense, via Middle English phrases such as god chep "favorable bargain" (12c., a translation of French a bon marché).

Sense of "lightly esteemed, common" is from 1590s (compare similar evolution of Latin vilis). The meaning "low in price" was represented in Old English by undeor, literally "un-dear" (but deop ceap, literally "deep cheap," meant "high price").

The word also was used in Old English for "market" (as in ceapdæg "market day"), a sense surviving in place names Cheapside, East Cheap, etc. Related: Cheaply. Expression on the cheap is first attested 1888. Cheap shot originally was U.S. football jargon for a head-on tackle; extended sense "unfair hit" in politics, etc. is by 1970. German billig "cheap" is from Middle Low German billik, originally "fair, just," with a sense evolution via billiger preis "fair price," etc.