An act when someone is in the middle of a conversation and the other party whips out their cell phone, cutting the other participant off, and showing the back of their phone. Figuratively being "slapped" by the phone. Hence the word "cell slapped". A bit worse than a pimp slap.
Another way of saying 'You will be sued' or 'I am going to get my lawers'
Mostly known from Danny Gonzalez's second channel '2 Danny 2 Furious' video 'getting slapped by legal :/'