Bestfriend (male) told me he's claimed me and I'm his. What does he mean?

This happened a couple days ago and I already tried asking him what he meant and he deflected and changed the subject.
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So I should have posted this beforehand but we were joking around and I ended up calling him a butthead and he called me one as well after that I said so? apparently you like it. And after that he responded with that's because you're mine, your my butthead and I claimed you.
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So I managed to get an answer out of him today completely blew me away. He said hed claim me as in seek out a relationship with me.
Bestfriend (male) told me he's claimed me and I'm his. What does he mean?
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