Fanny/ˈfæni/ Definition 1 of 4Pro Tip 1/3Amusing Slang Considered humorous and informal when talking about someone's rear end. When Joe fell, everyone chuckled, saying he hurt his fanny. Pro Tip 2/3Informal Settings Only Use 'fanny' casually with friends or family, not in formal situations. I wouldn't mention you bruised your fanny at the job interview. Pro Tip 3/3Potential Offense Be careful, as 'fanny' can be seen as crude or too personal in certain contexts. Aunt Martha seemed shocked when I said the cat sat on my fanny.