Green’s Dictionary of Slang

shill n.2

1. [late 19C+] (also shillaber, shiller, shilliber, shilliver) any form of criminal who poses as a member of the public to lure victims, usu. into confidence tricks.

2. [1910s] (US Und.) an apprentice criminal.

3. [1930s+] (gambling) a house player in a casino.

4. [1930s+] a promotion or, used fig., anyone who advocates something enthusiastically; thus also a promotion.

5. [1940s+] a member of the three-card monte team who appears to be another innocent gambler and who lures players into the game.

6. [1980s] a confidence trick.

7. [1990s+] a spokesperson – the implication is of mendacity – for an institution.

In phrases

shill through (v.) [abbr. shillaber, one who publicizes a circus, carnival etc.]

[1920s] (US Und.) to get in to an entertainment free.