1.) to have sex with one's balls. 2.) masturbating the penis on a scrotum, often with the aid of a water-soluble lubricant. 3.) placing your penis on a pair of balls while one nut is flopped over the top of the penis creating a ball tunnel in which to have sex with.
My boyfriend's asshole is on the rag so we had to ball fuck.
Last night my boyfriend was riding me reverse cowgirl and pointed his boner downward so he could ball fuck me.