To tell someone to use their
imagination. In effect, most people, at least normal, non-homoerotic sadists, wouldn't know what a
split cock looks like, so they'd have to use their imagination.
"Hey Jimmy, I heard that there's going to be a comic
book about
gay school children
trapped in Hell, what do you think that'll be like?"
"Splitcock."
"What?"
"Use your imagination fucktard."