I don't know if it gets to lunfardo, but it's an Argentinian for pimple, acne, comedón, barrito or any very notorious grain on the face. There is the myth that they appear because of a lack of sexual intercourse, and 'porn' would be a reduction of 'by not fucking'.
* Only one "like" per meaning and day, the more "likes" the meaning will appear higher in the list
