As far as I can tell, אף על פי/ Af Al Pi would literally translate to "nose on (my) mouth". Similarly, אף על גב/ Af Al Gav would literally translate to "nose on back". In actuality, they both mean "even though". Do mefarshim note this, and explain why this expression is worded the way it is?
From a quick sefaria search, it seems like the earliest source we have for either version is Tannaic - Mishna for Af Al Pi, Sifra for Af Al Gav.
מענה איש
there.)