To "smack your melon" means to hit your head, or noggin on something hard enough to cause a negative physiological reaction, such as a concussion, any cut or abrasion on your scalp, seeing stars, or even just getting a noticeable headache thereafter. It is usually caused by an inadvertent or unexpected action, such as tripping and falling down, being the victim of a noggin knocker, hitting your head on the dashboard or steering wheel of a car during a crash, or even misjudging the height of a tree branch, or some other low-hanging object, and stupidly hitting your top knot against it while passing by.
Hillary proved that if you smack your melon, you will not have to testify, at least until you recover from the ensuing concussion.
by Trochilus June 8, 2014