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.
"girl or woman whose attractiveness defies standards of beauty," 1849, a French expression (by 1780 in French), from fem. singular of joli "pretty" (see jolly) + laid "ugly," from Frankish *laid (see loath (adj.)).
Of beauty, as we narrowly understand it in England, [the 18c. French woman of society] had but little; but she possessed so many other witcheries that her habitual want of features and complexion ceased to count against her. Expression redeemed the absence of prettiness and the designation jolie laide was invented for her in order to express her power of pleasing despite her ugliness. ["The Decadence of French Women," Blackwood's Edinburgh Magazine, October 1881]
Among words for this concept, ugly is unusual in being formed from a root for "fear, dread." More common is a compound meaning "ill-shaped" (such as Greek dyseides, Latin deformis, Irish dochrud, Sanskrit ku-rupa). Another Germanic group has a root sense of "hate, sorrow" (see loath). Ugly duckling (1877) is from the story by Hans Christian Andersen, first translated from Danish to English 1846. Ugly American "U.S. citizen who behaves offensively abroad" is first recorded 1958 as a book title.