shill: someone who entices people into a carnival show or gambling game by posing as an enthusiastic customer
The origin of this word is unknown, but it first appeared in the United States around the beginning of the twentieth century. The original form may have been shillaber. Shill can be a noun, as in this sentence from the 1935 novel Honey in the Horn: "She had often thought of renting him out as a shill for some tent-show evangelist." It can also be a verb, as in this line from the 1948 novel of carnival life The Dead Ringer: "She was going to shill on Walter's wheel."
Lately the word has taken on the political and business meaning of one who poses as a disinterested advocate while actually being a partisan. A typical example comes from the July 12, 1976 U.S. News & World Report: "The former California Governor . . . 'has no desire to be a shill for Ford.'"
For a related carnival term from a slightly earlier time, see ballyhoo.