पिष्ट means something that is ground and पेषण means to grind. Means anything that is ground (flour) and पेषण means an act (done again and again) of grinding. Thus an act of grinding is called पिष्टपेषणन्याय useless repetition. Generally, anything that is ground does not need to be ground again, still if somebody makes a useless attempt like grinding the ground thing, it is simply a waste of time and ability. To sermonise this reality this Nyaya became prevalent.