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1.
The two lines and the dent formed by them, under ones nose extending to the top of the upper lip.

Son of a Bitch!! That coffee just burned the shit out of my philtrum.
by Richard Kirshen October 28, 2003
 
2.
The groove in the upper lip
A philtrum; what my FAS friend lacks
by Eviscerati December 09, 2008
 
3.
The word philtrum has 2 main meanings:
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.
1: You:"hi Steph! How you been?
Steph:"philtrum" *pokes your philtrum. walks away*

2: You:"hey sweety!"
Boyfriend:"her my giiiirl. Guess what I learnt today?"
You:"what?"
Boyfriend:"how to philtrum"
by *Angela lolly* February 20, 2013