(literally) A person who buys and sells horses, especially one who makes such transactions in a clever or skillful manner.
(figuratively, by extension) One who is skillful in transacting or negotiating of any kind.
1937, "Judiciary: Visibility Poor," Time, 22 Feb.:The three subordinates on whom President Roosevelt depends to win his Senate victories are Majority Leader Joseph T. Robinson of Arkansas, the bull-voiced, heavy-fisted field commander; Pat Harrison of Mississippi, the shrewd committee and cloakroom horsetrader; and James F. Byrnes of South Carolina.
