污

Three-drops water radical on the left - the side-form of 水. Indexes 污 in the water family. Originally 污 named a stagnant pool - water that doesn't move turns foul - from which came the metaphorical senses 'dirty, filthy, corrupt, pollute' (污染 pollution, 贪污 corruption). The water radical points to the literal scene.

Right 亏 supplies the sound - kuī shifts to wū through a long but attested k-/w- alternation in this phonetic family (also visible in 圬 wū 'to plaster'). Don't read 'deficient' into the meaning; the link is purely auditory and reaches back to Old Chinese reconstructions.