Old English blis, also bliðs "bliss, merriment, happiness, grace, favor," from Proto-Germanic *blithsjo (cognates: Old Saxon blidsea, blizza), from *blithiz "gentle, kind" + *-tjo noun suffix. Originally mostly of earthly happiness; influenced by association with bless and blithe.

often with out, by 1973, U.S. colloquial, from bliss, n..