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.
their praise was extravagant and insincere
a charming but thoroughly insincere woman
insight
insightful
insignia
insignificance
insignificant
insincere
insincerity
insinuate
insinuating
insinuation
insipid
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