Despite sounding like a rough burger you might pick up at your favourite hang out, mcjizzy is a term used to describe the act of getting in with a friend and due to overexcitement and loss of control, jizzing your pants.
"She stuck her tongue down my throat, and left me in a right Mcjizzy moment"
"oh shit! i think i'm about to do a mcjizzy"
"mmmm Katharine, uh uh MCJIZZY"