Ya lil nasty is a phrase you say to someone when they are being shady or bitchy. If you say Ya lil nasty to a person they are either excluding you, talking shit about you or being an asshole.
friend 1: omg guys i have to tell u something!
friend 2: tell us!
you: omg what is it?!
friend 1: oh not u only (friend 2) sorry!
you: Ya lil nasty.
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.