An exclaimation, generally used to describe either something surprising or something extremely cool, radical, awesome, or any other adjective from the Ninja Turtle vocabulary.
You have your hair cut, coloured and styled in a top salon by a man wearing more make-up than you. He tells you the cost of your new barnet - you gasp, then mutter "'sweet italian fuckingjesus!' I won't be coming back here again"
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.