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A personality trait that defines a grown up who acts or becomes very youthful or possibly childish when it comes to certain things i.e. Favourite foods.

A kid at heart is one who is usually not too serious.
Girl: "Hey, I got your favourite cake again."
Guy: "Yessss, thank you lol!"
Girl: "Haha, you're such a kid-at-heart!"
by MrMagnificentMedia September 15, 2013
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