duplicitous

The Latin 'duplex' meant simply twofold. Architects used it for two-story buildings; merchants used it in contracts referring to double weight or double measure. The word combined 'duo' (two) with 'plicare' (to fold), so a duplex thing was literally something folded in two. There was nothing sinister in the original geometry.

The moral turn came in Late Latin ecclesiastical writing. Christian theologians needed a word for the person who showed one face in public and another in private, who professed one belief while practicing another. Writers of the 5th century used 'duplicitas' to condemn the double heart, the soul that was not single before God. The innocent architectural term had acquired a conscience.

The noun 'duplicity' entered English around 1420, borrowed from Middle French 'duplicité,' and for the next five centuries it did the work alone. The adjective slot was filled by phrases: 'full of duplicity,' 'marked by duplicity,' 'a man of duplicity.' Then, in the mid-20th century, American English finally formed the adjective: duplicitous. It had waited an unusually long time.

The delay is worth noting. English had 'duplicity' since the 15th century but formed the adjective only when political language began to want a one-word modifier. Cold War rhetoric, diplomatic criticism, and the vocabulary of character assessment all found uses for a word that accused not just a single lie but a systematic doubleness maintained across time.