1880, present participle adjective (from the first typically with idiot) from blither (v.) "to talk nonsense." From 1872 as a verbal noun.
1868, variant of blether "talk nonsense," 1520s, a northern British and Scottish word, from Middle English blather (see blather (v.)). Related: Blithered; blithering.
Senseless or incoherent: blithering idiot (1889+)