For those wondering, this is actually a thing, according to Urban dictionary.
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Arm Sex
1. To hug someone
2. When your arm rubs up against someone else's arm (on purpose or accidentally, doesn't matter)
Note: May also be called an "arm f***" or "arm ****ing".
Girl: Are they dating?
Boy: Just because they have arm sex on a regular basis doesn't mean they are!
Girl: Oh, just friends then?
Boy: Say... can we have arm sex?
Girl: Ew, no.