The term "Bowsaw" originated in the North East of Ireland, in a small rural town named Dundalk. The word has been used in the context of sex, that is to saw - "did you give her a bit of bowsaw boi?" namely in the context of the doggy style position. The word is also closly linked to "wavin" which is another Irish word for the penis.
After bating it into a young one and on the walk of shame or fame home and you run into one or two of the lads, they may ask you - Did you give her a bit of bowsaw boi?
"Did you give her a bit of wavin boi, a bit of BOWSAW!!!"
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.
FRIEND A: "Did you just take a stealthie of me?"
FRIEND B (turning phone around): "no I was just using snapchat's new filter, see?"