Gaping for gudgeons. Looking out for things extremely improbable. As a gudgeon is a bait to deceive fish, it means a lie, a deception.
Make fools believe in their foreseeing Of things before they are in being, To swallow gudgeons ere they're catched, And count their chickens ere they're hatched.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894