loathly adj.

Old English lað "hated; hateful; hostile; repulsive," from Proto-Germanic *laitha- (source also of Old Saxon leth, Old Frisian leed "loathsome," Old Norse leiðr "hateful, hostile, loathed;" Middle Dutch lelijc, Dutch leelijk "ugly;" Old High German leid "sorrowful, hateful, offensive, grievous," German leid "hateful, painful"), from PIE root *leit- (1) "to detest."

And niðful neddre, loð an liðer, sal gliden on hise brest neðer [Middle English Genesis and Exodus, c. 1250]

Weakened meaning "averse, disinclined" is attested from late 14c. "Rare in 17th and 18th cents.; revived in the 19th c. as a literary word" [OED]. Loath to depart, a line from some long-forgotten song, is recorded since 1580s as a generic term expressive of any tune played at farewells, the sailing of a ship, etc. French laid, Italian laido "ugly" are from the same Germanic source. The sense "ugly" persisted in English into 15c. in the marriage service, where a man took his wife for fayrer, for layther. Related: Loathness.

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.