"long, lash-like appendage," 1837, from Latin flagellum "whip, scourge," also figurative, diminutive of flagrum "a whip," from PIE root *bhlag- "to strike" (cognates: Latin flagitium "shameful act, passionate deed, disgraceful thing," flagitare "to demand importunately;" Old Norse blakra "to flutter with the wings," blekkja "to impose upon;" Lithuanian blaškau "to and fro").