The words ugli, ugly sound the same but have different meanings and spellings. Why do ugli, ugly sound the same even though they are completely different words?
The answer is simple: ugli, ugly are homophones of the English language.
A cross between a tangerine and grapefruit, grown in the West Indies.
Informal One that is ugly.
Marked by or inclined to anger or bad feelings; disagreeable: an ugly temper; an ugly scene.
Likely to cause embarrassment or trouble: "Public opinion in both nations could take an ugly turn” ( George R. Packard).
Threatening or ominous: ugly black clouds.
Definitions from Wiktionary, Creative Commons Attribution/Share-Alike License, from The American Heritage® Dictionary of the English Language, 4th Edition and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.