mid-13c., uglike "frightful or horrible in appearance," from a Scandinavian source, such as Old Norse uggligr "dreadful, fearful," from uggr "fear, apprehension, dread" (perhaps related to agg "strife, hate") + -ligr "-like" (see -ly (1)). Meaning softened to "very unpleasant to look at" late 14c. Extended sense of "morally offensive" is attested from c. 1300; that of "ill-tempered" is from 1680s.
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.
"city ruffian, one of a gang who assaulted people and property in mid-19th century American cities," 1856, originally in Baltimore, from plug (n.), the American English slang name for the tall, silk stovepipe hats then popular among young men, + ugly. Sometimes as the name of a specific gang, but often generic.
early 13c., "to inspire fear or loathing;" mid-14c. "to feel fear or loathing," from Old Norse ugga "to fear, dread" (see ugly). Related: Ugging.