"You weren't at the party when it was invented". A word used when you have made a new word and nobody knows what that word was so you just say/write "YWATPWIWI".
what-pwee-wee (for those having trouble pronouncing it, silent Y)
Valerie: Hi John, you were so funny the other night I was Pmslsmicb
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.