Flesh radical on the left - visually identical to 月 but semantically the meat/anatomy form, used for body substances. This puts 脓 in the bodily-discharge family alongside 脏 organ, 腐 rot, 肿 swell. Pus is, after all, infected tissue.
Right side supplies the sound: nóng → nóng (exact match). 农 is purely phonetic; its 'agriculture' meaning is not active. The traditional form 膿 used the fuller 農 phonetic; simplification swapped both sides to their shorter forms.
