Moody means changing quickly from cheerful to grumpy without warning. A moody teenager, he is moody, moody as anything.
It describes unpredictability rather than any particular mood, which is what makes it a complaint. Of music, film or light, moody is a compliment meaning atmospheric and slightly dark: moody lighting, a moody soundtrack. Note mood, which also has a grammatical sense — the indicative and subjunctive moods; and that moody comes from an Old English word meaning brave, which has entirely reversed.
1adjectivemoodyChanging quickly from cheerful to grumpy.
“Teenagers can be moody, happy one minute and slamming doors the next.”
2adjectiveatmosphericAtmospheric and slightly dark.
“The photographer took moody black-and-white pictures of the foggy harbour.”
moody is a quality of changeable temper.
Old English modig meant brave or high-spirited, from mod, the mind or courage. The word has slid from courageous through spirited to bad-tempered, which is a fairly complete reversal in a thousand years.
Proto-Germanic
*mōdagaz — courageous
Old English
mōdiġ — brave
Middle English
mody
English, today
moody
From Middle English mody, modi, from Old English mōdiġ (“brave”), from Proto-Germanic mōdagaz (“courageous”). by surface analysis, mood + -y.
Etymology from Wiktionary, CC BY-SA