The word was first coined in the infamous 1968 film "Danny and the Postmaster". Often associated with homosexuality although equally applicable to heterosexuality, a poosterini is a rarely washed penis encrusted in feces. According to the movie, the creation of a poosterini requires complete neglect of hygiene towards the penis for a period of at least three weeks combined with consistent anal sex.
From "Danny and the Postmaster": "Danny, are you familiar with the term, poosterini?"
by HarryHaller June 21, 2009