Boyfriend gets pissy when I called him a brat?

lanadelrey25
I joke around with him all the time and call him dumb, old, nerdy etc and he calls me names back but all in good fun. Then one day I call him a little brat and he says "can you take it back" and I thought he was kidding but he kept asking me to take it back so I did.
Then today it slipped out and I called him a brat again and he said "can you take it back" so I said in I guess a condescending baby voice "ok I'll take it back" but I was still joking around thinking he was, and he gets really sensitive and pissy... like wtf.
I don't understand why that pisses him off so much, and I asked if that's what someone else used to call him and he said no so I don't understand.

My boyfriend when I call him that:
Boyfriend gets pissy when I called him a brat?
Boyfriend gets pissy when I called him a brat?
me:
Boyfriend gets pissy when I called him a brat?
I mean obviously I'm definitely not gonna call him a brat anymore.
He's lying and it has to do with his childhood
He's too sensitive about that word for no reason
Other
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Boyfriend gets pissy when I called him a brat?
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