Filthy pelf. Money. The word was anciently used for refuse or rubbish. “Who steals my purse steals trash.” Filthy means ungodly; the Scripture expression is “unrighteous mammon.” It is certainly not connected with pilfer, as it is usually given; but it may possibly be with the Anglo-Saxon pila, a pile or heap. The old French word pelfre means spoil.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894