Disappoint is a synonym of foil.

As verbs the difference between disappoint and foil

is that disappoint is to displease by eg underperforming or disappoint can be (internet slang) while foil is to prevent (something) from being accomplished or foil can be (mathematics) to multiply two binomials together or foil can be (obsolete) to defile; to soil.

As a noun foil is

a very thin sheet of metal or foil can be failure when on the point of attainment; defeat; frustration; miscarriage or foil can be (hunting) the track of an animal.