Humorous misspelling of the word "penis." Comes from a translation of a Fate/Stay Night parody comic, where the translators kept the Japanese convention of using circles to edit out certain letters of certain words; the self-censored term for "penis" thus appeared as "p•n•s", and conceivably as "ponos," "pOnOs" or "p0n0s" - all variations which have been used on the internet. Usually used in conjunction with another similarly censored term, vagooo.
Put your ponos up to my vagooo and j-JAM IT IN! (Actual usage from the original Fate/Stay Night parody)