Green’s Dictionary of Slang

shim n.2

[SE shim, a sliver of metal, used to fill the space between parts of machinery that are subject to wear]

a piece of plastic used to open a door.

Show quotations
L. O’Donnell Face of Crime 12: The lock was of the deadbolt type that doesn’t yield to the opportunist’s plastic shim .
R. Parkes Guardians 8: Had the door fitted flush to the frame, the old perspex shim wouldn’t have slipped in .
[US]J. Wambaugh Glitter Dome (1982) 29: The police identification cards couldn’t even get a check cashed [...] But at least they could slip a lock better than most shims.