Definition
To be reprimanded or called to account by someone in authority.
Under consideration or discussion.
How People Actually Use It
A slice on the mat is a Second War British term. It signifies the eager anticipation of a serviceman returning home on leave; as his woman opens the door they can't even wait to get to the bedroom, and have sex there and then, on the mat, just inside the door. It is still used.
"I hadn't seen her for three months, and when I turned up she immediately gave me a slice on the mat. She did remember to close the door, though. . ."
Source: Urban Dictionary