This emoticon is meant as a reference to an expression made by the popular My Little Pony: Friendship is Magic character Rainbow Dash, who made the expression in response to the accomplishments of Applejack, another character of the show, in the episode "Applebuck Season" of the shows first season. It is meant as a textual representation of extreme happiness or approval.
Chat Partner 1: Hey bro I got promoted at my job today!
Chat Partner 2: Congratulations dude! /)^3^(\