The analog smiling face.
It's used either as:
A. an alternative to the emojis
B. a more psycho-like smile (as the bracket gives it an unnaturally wide grin)
Bob: Tommy, where has Jake gone?
Tommy: :)
Bob: I'm calling the police.
A synonym for "ass", applicable when "ass" is used to express quantity of a trait.
Bro, your homie is a goofy ahh fella.