The Collaborative International Dictionary of English v.0.48:
Racker \Rack"er\ (r[a^]k"[~e]r), n. 1. One who racks. [1913 Webster] 2. A horse that has a racking gait. [1913 Webster]WordNet (r) 3.0 (2006):
racker n 1: an attendant who puts pool or billiard balls into a rack