Verb (5-dan, う, intransitive)
1. to laugh
彼は楽しそうに笑っていた。He was laughing happily.
2. to smile
彼は薄く笑っている。He is smiling faintly.
Verb (5-dan, う, transitive)
3. to sneer; to ridicule
esp. 嗤う
Verb (5-dan, う, intransitive)
4. to be dumbfounded; to be flabbergasted
as 笑ってしまう or 笑っちゃう