c.1300, "to trick, beguile," perhaps from O.Fr. japer "to howl," of echoic origin, or from O.Fr. gaber "to mock, deride." Phonetics suits the former, but sense the latter explanation. Took on a slang sense mid-15c. of "have sex with," and disappeared from polite usage. Revived in harmless M.E. sense of "say or do something in jest" by Scott, etc.