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.

bitch-slap v.
to slap someone with an open hand, esp. in an attempt to put them in their place or cause humiliation; from black English, slapping a person as a pimp would slap a prostitute; vulgar slang; bitch-slapped, bitch-slapping; also used figuratively