When your jaw AKA mouth can no longer open because of too much sexual acvitity.
When a mouth is fucked so many times by a big cock that it can no longer stay open for further pounding.
"Apparently Debbie died of Fuck-Jaw!"
"Once again I was on another date with some bitch called Susan... the ho' couldnt take my big cock, and the dumb bitch got Fuck-Jaw after 2 hours of jackhammering her mouth!" "So anyhow I just bent her over and fucked her in the ass!"
"I heard Glenn came down with Fuck-Jaw in Miss Anderson's Drama class...Hrmm I guess hetro status may no longer be guarenteed!"