What does it mean when a guy lightly brush against a girl's sleeve when passing by her?

There is this guy who is in two of my college classes. In one of my classes he sits on the same row as me. I am quiet and we hardly never talk. If he does say something, just to ask me a question about class. When he passes by me, he will lightly brush against my sleeve. He has done this more than one time. There be room for him to walk.

I was sitting at another place on campus and he lightly brushed against my sleeve, arm. I would like to know what does it mean when a guy lightly brush against a girl's sleeve, arm when passing by her?
What does it mean when a guy lightly brush against a girl's sleeve when passing by her?
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