When a judge unleashes a verbal smackdown of someone in a courtroom setting. A bench slap can be directed at anyone in the courtroom for example: an attorney, a plaintiff, a defendant, etc.
A benchslap is when a judge humiliates an attorney, insults another judge, or reverses a lower court in a particularly demeaning manner. A judicial bitch slap, if you will. This term was popularized by David Lat of AboveTheLaw.com (when he was blogging for UnderneathTheirRobes.com).
That was a classic Kozinski benchslap.
Judge Judy totally benchslapped that hooker that claimed to be a model, did you see that?
She might be a litigatrix, but that didn't help her dodge that brutal benchslap.