A Greek who induced the Trojans to receive the wooden horse. (Virgil: AEneid, ii. 102, etc.) Anyone leceiving to betray is called “a Sinon.”
And now securely trusting to destroy,
As erst false Sinon snared the sons of Troy.
Camoëus: Lusiad, bk. i.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Sinon from Fact Monster: