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"having a slippery surface," c.1500, from Middle English sliper (adj.) "readily slipping," from Old English slipor "slippery, having a smooth surface" (see slip (v.)) + -y (2). Metaphoric sense of "deceitful, untrustworthy" is first recorded 1550s. Related: Slipperiness. In a figurative sense, slippery slope is first attested 1844. Slippery elm (1748) so called for its mucilaginous inner bark.