From Middle English (14th century), from Scandinavian 'glome' ("to stare somberly").
Partial or total darkness; a state of depression or despondency; a dark or shadowy place.
From Middle English 'gloumen' (to look sullen, to frown), probably of Scandinavian origin; compare Norwegian dialectal 'glome' (to stare somberly). The word began as a verb describing a facial expression — a dark, sullen look — before extending to describe darkness itself and then emotional darkness. The progression from 'sullen face' to 'dark mood' to 'literal darkness' is an interesting reversal: usually words go from physical to metaphorical, but 'gloom' went from emotional expression to physical atmosphere. Key roots: glome (Norwegian dialectal: "to stare somberly, to look murky").
'Doom and gloom' — the famous phrase — is actually a modern reversal. The original 1950s coinage was 'gloom and doom' (from the musical 'Finian's Rainbow,' 1947), but speakers naturally flipped it to 'doom and gloom' because English prefers the shorter vowel first, a pattern linguists call the 'ablaut reduplication rule.'