From Proto-Indo-European *plúsis (“flea”), with metathesis plus → pusl → pūl, with a suffix -ex, -icis also found in cīmex (“bug”) and culex (“gnat”). Cognates include Ancient Greek ψύλλα (psúlla), Sanskrit प्लुषि (plúṣi), Old Armenian լու (lu) and Old English flēah, flēa (English flea).