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"one who acts as a decoy for a gambler, auctioneer, etc.," 1916, probably originally circus or carnival argot, probably a shortened form of shillaber (1913) with the same meaning, origin unknown. The verb is attested from 1914. Related: Shilled; shilling.
: That summer he shilled for a sidewalk hawker (1914+)
[origin unknown; perhaps, since it is a shortening of shillaber, ultimately fr Shillibeer, the name of an early 1800s British owner of a large bus company, the reference being to persons hired as decoys to sit in buses and attract passengers]