This term is in fact used to describe a non-existant condom. It is meant to insult really dumb-ass girls who would believe that a guy actually has created this invisible condom which will prevent her from becoming the slut who got preggo (pregnant) in high school. ATTENTION: the invisible condom does not exist and can not stop the load!
"Stacy I'm pregnant?"
"I don't know... Mitch put on his invisible condom"