Hitting harder than necessary. Working stiff means your strikes, kicks, and throws are landing with real force. Sometimes it's a stylistic choice — Japanese strong style is intentionally stiff. Sometimes it's a wrestler being careless or sending a message. Getting stiffed in a match is unpleasant. Getting stiffed deliberately is a receipt waiting to happen.
"That chop was stiff. You could see the handprint from the nosebleeds. That wasn't a work, that was a message."