What does it mean when my boyfriend press on his penis?

Sometimes when we’re just sitting casually he presses on his penis with the bottom of hands (the part of the hand next to the fist) .. what does it mean? Sometimes it happens when we’re casually talking and sometimes when I say sth cute or when we’re flirting.. but then again he does it at other randomness times? So guys why &/when do you do that?

What does it mean when my boyfriend press on his penis?
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