Taking this excess of gas back to Proto-Indo-European, philologists reconstruct two roots for 'fart': *perd- 'to fart loudly' (*prdi-s whence OE feortan and Welsh rhech 'fart') and *pezd- 'to pass wind softly' whence Greek bdeo, Latin pedo 'to fart', podex 'butt, behind', pedis 'louse'. The Latin podex also gives the German words der Podex and der Popo 'butt'.