sweater on the rail
n.— «Hustling doesn’t merely involve the players at the table. There was a rogue’s gallery of “stakehorses” (financial backers), “sweaters on the rail” (side bettors) and “nits” (kibitzers). As the gambling spigot has been turned off, the local poolroom—once a civic institution—has almost vanished.» —“Jump the Shark” by L. Jon Wertheim New York Times Nov. 24, 2007. (source: Double-Tongued Dictionary)