The abbreviation for "See you next week"
C = See
U = You
N = Next
7 = Week (there are seven days in a week)
Friend: "hey bro, im gonna have to get going"
You: "Aight man cun7"
Friend: "Wtf did you call me!?!?"
You: "lololol, its short for 'see you next week', you tard"
Friend: "ah true, catch"