Not true, stuff, make-up. (Gaelic, ffug, deception; Welsh, ffug, pretence; whence ffugiwr, a pretender or deceiver.) A word of contempt bestowed on one who says what is absurd or untrue. A favourite expression of Mr. Burchell in the Vicar of Wakefield.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894