A fairly common dermatological condition consisting of a rash or an outbreak of acne on ones backside. Not limited to the back, but anywhere that is not considered someone's "front".
Even those who've never suffered from backacne should immediately understand what it is from it's use in context.
Two syllables, and is often mis-heard in advertisements for arthritis nostrums: "...to cure back, knee, hip, and joint pain."
"Hey, could you put some of this Persa-Gel on my back? I've got an outbreak of backacne and I can't reach it."
Flavor Flav had backacne,
just ask James Cagney,
he beat up on a guy
when they found he was a fag-ny.
(sung to "Letter To The New York Post" by 'Public Enemy')