1: noun. The little groove between the bottom of your nose and your top lip. I have no clue what it is there for, but now you know what it is called.
2: verb. The art of performing oral sex while shaving. This originated because some kid at my school with a dirty mind thought it was 'full trim'. The term has since caught on and is now widely recognised.
Steph:"philtrum" *pokes your philtrum. walks away*
2: You:"hey sweety!"
Boyfriend:"her my giiiirl. Guess what I learnt today?"
Boyfriend:"how to philtrum"