shim n.

in mechanics, "a thin strip used to fill up space caused by wear, or to prevent it," 1723, apparently a Kentish word, of unknown origin. The same word appears older in a sense of "piece of iron fitted to a plow for scraping soil." The carpentry sense "thin slip of wood to fill up a space or raise a level" is by 1860. In criminal slang, in reference to a type of thin, flat tool or device used to pop a simple door lock, by 1968.

"to wedge up a surface or fill out by means of a shim," 1877, from shim (n.). Related: Shimmed; shimming.