A ball of hair that forms in the stomach of an animal (especially cats) when they groom themselves, or sometimes in plumbing from collected hair.
A simple compound of 'hair' (from Proto-Germanic 'hēr') and 'ball' (from Old Norse 'bollr'). This is modern English, becoming common only in the 20th century as pet ownership increased and veterinary language became more standardized.
Cats cough up hairballs because they're fastidious groomers, but ironically, their amazing cleaning behavior creates this gross problem—it's a funny reminder that evolution doesn't optimize for everything, and even the 'best' systems have annoying bugs.
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