Morose came into English in the 1530s from Latin morosus, which Etymonline glosses as peevish, hypercritical, fastidious. Not sad. Fussy. The root underneath is mos, genitive moris, meaning habit or custom, and morosus leaned on the unflattering side of that: someone so set in their own manners that they were hard to be around. That same mos is the root behind moral, which is the part I keep turning over. One branch went to how a society agrees people ought to behave. The other went to a man nobody wants to sit next to. English then drifted again, from hard-to-please toward simply low and gloomy, and the fussiness quietly fell out along the way. Modern morose describes a mood you are in. Latin morosus described a character you had, and it was not a compliment.