"emotional condition, state of mind as regards passion or feeling," c. 1300, from Old English mod "heart, frame of mind, spirit; courage, arrogance, pride; power, violence" (also used to translate Latin animus, mens), from Proto-Germanic *mōda- (source also of 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"), a word of unknown origin (Boutkan finds no acceptable IE etymology).
A much more vigorous word in Anglo-Saxon than currently, and used widely in compounds (such as modcræftig "intelligent," modful "proud"). The Old English senses now are obsolete. Meaning "a fit of bad temper; sullenness, sudden anger" is by 1859. To be in the mood "in a state of mind to be willing (to do or omit something)" is from 1580s. First record of mood swings is by 1939.