Guys, What does it mean when a guy bucks their hips back and forwards right in front of your eyes?

One of my guy friends is every sexual and he bucked his hips as if he's f***king someone and does moaning noises to the air. And he does this in front of me and I sometimes was like wondering why I'm even friends with this dude...
please tell me why he does this? It's weird
Guys, What does it mean when a guy bucks their hips back and forwards right in front of your eyes?
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