A reliable etymology would be hard to suggest in view of the graphic ambiguity.
Akkadian
paˀāṣu - to cleave, to crush, to grind
(CAD P 1, AHw. 807)
Var.: pêṣu.
Von Soden’s comparison with Arb. fḥḍ “to crush fresh, humid substances, such as a melon” (LA 7 228) is reasonable, although the Arabic verb is rare.