From Old Norse 'uggligr' (dreadful), from 'uggr' (fear) — originally terrifying, not merely unattractive.
Unpleasant or repulsive in appearance; disagreeable to look at.
From Old Norse 'uggligr' meaning 'dreadful, fearsome,' derived from 'uggr' (fear, dread). The word entered Middle English with a sense closer to 'frightening' than to 'aesthetically unpleasant' — something ugly was originally something that inspired fear or horror. The semantic narrowing from 'dreadful' to merely 'unattractive' occurred gradually
Swedish 'uggla' (owl) comes from the same Norse root as English 'ugly' — the owl was named for being the 'dreadful' or 'fear-inspiring' bird of the night. The English word originally meant 'fearsome,' not just 'unattractive,' so calling someone ugly in 1250 was saying they were terrifying, not merely plain.