Making an educated guess.
Extrapolating from previous experience and personal knowledge either to predict future events or explain events about which you have no direct knowledge.
"Why did she ignore me just now?"
"I'm just fishing off the bridge here, but I think it's because you didn't call her last time."
Man: Sometimes it's hard to resist the urge to drive right off a fucking bridge HHHhahaha oh look there's one
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when you're holding up your phone and making faces at it, as though you are taking a selfie, but you're really taking a picture of the person across from you or the wall or anything else that seems interesting but you don't want to be caught dead taking a picture of.
This action is often made more convincing by wiggling the eyebrows or opening the mouth, to pretend you're trying to get a Snapchat filter to work.