Insincere

google

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mid 17th century: from Latin insincerus, from in- ‘not’ + sincerus ‘sincere’.

wiktionary

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From Latin insincerus.

etymonline

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insincere (adj.)

1620s (implied in insincerely), from Latin insincerus "spoiled, corrupted; not genuine, not pure, adulterated," from in- "not" (see in- (1)) + sincerus "genuine, candid" (see sincere). Related: Insincerely.