What does it mean when my boyfriend calls me his buttercup?

Sofia1997
Like I noticed that when we're having sex he would grab my ass and say that's my ass and then he would make me say that my ass belongs to him which I love but 4 days ago when I saw him on our date he was talking to me and he said that your my buttercup and I was so happy but I don't know exactly how he means it like is he saying I'm his girl or something?
What does it mean when my boyfriend calls me his buttercup?
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