Origin: It is traditional in team sports, esp. all types of football, to slap the butt of one who scores a goal or point. Now often also common in athletics and golf.
Increasing becoming popular in business after a contract has been signed, or as way of congratulating a salesman or lawyer who has pulled off a good deal or got a good decision on a case.
2. "Mary, I won the Brinkman case! The boss gave me a real nice sportman's slap right on the seat of my Armani suit."