Swap
Middle English (originally in the sense ‘throw forcibly’): probably imitative of a resounding blow. Current senses have arisen from an early use meaning ‘strike hands as a token of agreement’.
wiktionary
From Middle English swappen(“to swap”), originally meaning "to hurl" or "to strike", the word alludes to striking hands together when making an exchange; probably from Old English *swappian, a secondary form of Old English swāpan(“to swoop”). Cognate with German schwappen(“to swap”). Compare also Middle English swippen(“to strike, hit”), from Old English swipian(“to scourge, strike, beat, lash”), Old Norse svipa(“to swoop, flash, whip, look after, look around”). More at swipe.
[1620] From the verb swap.
From Middle English swap, swappe(“a blow, strike, lash from a whip”), from the verb (see Etymology 1 above).
etymonline
swap (v.)
c. 1200, "to strike, strike the hands together," of uncertain origin, possibly imitative of the sound of hitting or slapping. The sense of "to exchange, barter, trade" is first recorded 1590s, possibly from the notion of slapping hands together as a sign of agreement in bargaining (as in strike a bargain). Related: Swapped; swapping. The noun in this sense is attested from 1620s; earlier "a striking, an act of striking" (mid-13c.). Swap-meet attested from 1968, American English.
