Batty means acting in a silly, eccentric, or slightly crazy way. If your teacher starts wearing mismatched socks and speaking in rhymes for no reason, your classmates might whisper that she's gone a bit batty. When someone calls an idea batty, they think it's absurd or wildly impractical, like a plan to train squirrels to deliver mail.

While calling someone batty can sound harsh, people often use it affectionately about harmless quirks, like an uncle who collects 500 rubber ducks or a neighbor who talks to her garden plants.

In British English especially, batty describes someone charmingly odd rather than truly unwell. You might read about a batty inventor in a story who creates bizarre but wonderful contraptions. The word carries a lighter, gentler tone than words like “insane” or “deranged.” When your friend suggests eating dessert before dinner and you say “That's batty!”, you're not genuinely worried about their mental health; you're just calling the idea delightfully ridiculous.