"sourly, with sullen austerity," 1650s, from morose + -ly (2).
Entries linking to morosely
morose adj.
1530s "gloomy, of a sour temper, sullen and austere," from Latin morosus "morose, peevish, hypercritical, fastidious," from mos (genitive moris) "habit, custom" (see moral (adj.)). In English, manners by itself means "(good) manners," but here the implication in Latin is "(bad) manners."
-ly 2
common adverbial suffix, forming from adjectives adverbs signifying "in a manner denoted by" the adjective, Middle English, from Old English -lice, from Proto-Germanic *-liko- (cognates: Old Frisian -like, Old Saxon -liko, Dutch -lijk, Old High German -licho, German -lich, Old Norse -liga, Gothic -leiko); see -ly (1). Cognate with lich, and identical with like (adj.).
Weekley notes as "curious" that Germanic uses a word essentially meaning "body" for the adverbial formation, while Romanic uses one meaning "mind" (as in French constamment from Latin constanti mente). The modern English form emerged in late Middle English, probably from influence of Old Norse -liga.