"offensively abusive, wantonly irreverent, coarse, obscene," of persons, conduct, speech, etc., c. 1500, from obsolete noun ribald, ribaud "a rogue, ruffian, rascall, scoundrell, varlet, filthie fellow" [Cotgrave], mid-13c. (late 12c. as a surname), attested from late 14c. in the sense of "one who uses offensive or impious language, one who jests irreverently."
This is from Old French ribaut, ribalt "rogue, scoundrel, lewd lover," also as an adjective, "wanton, depraved, dissolute, licentious," a word of uncertain origin, perhaps (with suffix -ald) from riber "be wanton, sleep around, dally amorously," from a Germanic source (compare Old High German riban "be wanton," literally "to rub," possibly from the common euphemistic use of "rub" words in venery, from Proto-Germanic *wribanan from PIE root *wer- (2) "to turn, bend").
Other early adjectival forms were ribaldous "riotous, unruly" (c. 1400); ribaldy (early 15c.).