"deceptiveness, character or practice of speaking differently of the same thing or acting differently at different times or to different persons," early 15c., from Old French duplicite (13c.), from Late Latin duplicitatem (nominative duplicitas) "doubleness," in Medieval Latin "ambiguity," noun of quality from duplex (genitive duplicis) "twofold," from duo "two" (from PIE root *dwo- "two") + -plex, from PIE root *plek- "to plait." The notion is "a state of being double" in one's conduct (compare Greek diploos "treacherous, double-minded," literally "twofold, double").
word-forming element making adjectives from nouns, meaning "having, full of, having to do with, doing, inclined to," from Old French -ous, -eux, from Latin -osus (compare -ose (1)). In chemistry, "having a lower valence than forms expressed in -ic."
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Definitions of duplicitous from WordNet
marked by deliberate deceptiveness especially by pretending one set of feelings and acting under the influence of another; "she was a deceitful scheming little thing"- Israel Zangwill; "a double-faced infernal traitor and schemer"- W.M.Thackeray;