Is it ensure, insure, or assure?
1650s, "thin, slight, slender," from Dutch slim "bad, sly, clever," from Middle Dutch slim "bad, crooked," from Proto-Germanic *slembaz "oblique, crooked" (cf. Middle High German slimp "slanting, awry," German schlimm "bad, cunning, unwell"). In English 17c. also sometimes with a sense "sly, cunning, crafty." Related: Slimly; slimness. With obsolete extended adjectival forms Slimsy "flimsy, unsubstantial" (1845); slimikin "small and slender" (1745). Slim Jim attested from 1887 in sense of "very thin person;" from 1902 as a type of slender cigar; from 1975 as a brand of meat snack.
To make neat and more attractive; furbish; gussy up: What are they all slicked up for? (1828+)