A term used by postmen (mailmen) in the U.K for mail that is purposely not delivered, usually for top floor flats.
This pratice is not endorsed by the Royal Mail but is common place amongst posties, especially on a Saturday in an effort to secure an early finish.
Slamming involves reintroducing the letter into the mail system the next day back at work.
The undelivered item is refered to as a slammer.
John the postie wants home for 10, he's going to be slamming all the crap for top floors today.