an unstressed syllable prefixed to onomatopoeic and other expressive words, usually forming adverbs or interjections: kerflop; kerplunk; ker-splosh.
Origin of ker-
perhaps < Scots dialect car-, cur-, currie- (as in carfuffle, carwhuffle to disarrange, carnaptious irritable, curriebuction a confused gathering, etc.), based on car, earlier ker left (hand or side) < Scots Gaelic cearr wrong, awkward, left-handed (compare MIr cerr crooked, maimed); variants without r probably reflect forms in r-less dialects
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
U.S. slang prefix, by 1836 as che-, 1843 as ker-, possibly from influence of German or Dutch ge-, past participial prefix; or ultimately echoic of the sound of the fall of some heavy body.
Online Etymology Dictionary, © 2010 Douglas Harper