Brewer's: Foil

That which sets off something to advantage. The allusion is to the metallic leaf used by jewellers to set off precious stones. (French, feuille; Latin, folium; Greek, phullon, a leaf.)

“Hector, as a foil to set him off.”

Broome.

I'll be your foil, Laertes. In mine ignorance Your skill shall, like a star i' the darkest night, Stick flery off indeed.

Shakespeare: Hamlet, v. 2.

He foiled me.

He outwitted me.

“If I be foiled, there is but one ashamod who never was gracious.” —Shakespeare: As You Like It, i. 2.

To run a foil.

To puzzle; to lead astray. The track of game is called its foil; and an animal hunted will sometimes run back over the same foil in order to mislead its pursuers.

Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894

Folio

Fo-hi