Meaning

adjective out of condition; not strong or robust; incapable of exertion or endurance

originFrom a variant of flappy, from flap (“to hang loose”). Compare English dialectal flapsy (“flabby”), Middle Dutch flabbe (“a slap in the face; a fan-blade; a hair ribbon; a wagging tongue”), Middle Low German flabbe (“a gaping mouth; a chatterbox”), Danish flab (“the jaw; cheeks; a malapert”), Swedish flabb, fläff (“the hanging underlip of an animal; guffaw; driveller”), German Flabbe (“a gob;…

Etymology from Wiktionary, CC BY-SA 3.0