Appeared suddenly in the 1770s, possibly blending 'flabby' (going limp with shock) and 'aghast' — origins genuinely uncertain.
To surprise greatly; to astonish utterly.
First recorded in 1772 in a British periodical as a colloquial term, origin obscure. The leading hypothesis is that it blends or compounds 'flabber' (possibly from 'flabby' — limp, slack, from Flemish 'flabbe', related to 'flap') + 'aghast' (struck with horror, from Old English 'gæstan', to frighten, from 'gāst', ghost or spirit, from Proto-Germanic *gaistaz, from PIE *gheis-, to be excited or frightened). The compound would convey the image of someone so shocked they go limp — flabby with astonishment — while simultaneously struck as if by a ghost. Alternative theories
Its expressiveness ensured its survival despite lacking classical credentials — English sometimes creates words by feel rather than by rule.