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"emotional condition, frame of mind," Old English mod "heart, frame of mind, spirit; courage, arrogance, pride; power, violence," from Proto-Germanic *motha- (cf. Old Saxon mod "mind, courage," Old Frisian mod "intellect, mind, intention," Old Norse moðr "wrath, anger," Middle Dutch moet, Dutch moed, Old High German muot, German Mut "courage," Gothic moþs "courage, anger"), of unknown origin.
A much more vigorous word in Anglo-Saxon than currently, and used widely in compounds (e.g. modcræftig "intelligent," modful "proud"). To be in the mood "willing (to do something)" is from 1580s. First record of mood swings is from 1942.
"grammatical form indicating the function of a verb," 1560s, an alteration of mode (n.1), but the grammatical and musical (1590s) usages of it influenced the meaning of mood (n.1) in phrases such as light-hearted mood.
mood 1 (mōōd)
A state of mind or emotion.