Shill is originally a carnival term for a fake customer who attempts to draw others into a show.
From Wikipedia:
A shill is an associate of a person selling goods or services who pretends no association to the seller and assumes the air of an enthusiastic customer. The intention of the shill is, using crowd psychology, to encourage other potential customers unaware of the set-up to purchase said goods or services. Shills are often employed by confidence artists.
The word "shill" is probably related to "shillaber", a word of obscure early-20th century origin with the same meaning.
Shills are illegal in many circumstances and in many jurisdictions because of the frequently fraudulent and damaging character of their actions. However, if a shill does not place uninformed parties at a risk of loss, but merely generates “buzz,” the shill's actions may be legal. For example, a person planted in an audience to laugh and applaud when appropriate, see "claque", or to participate in on-stage activities as a "random member of the audience", is a type of legal shill.
