Use the adjective disingenuous to describe behavior that's not totally honest or sincere. It's disingenuous when people pretend to know less about something than they really do.

Disingenuous combines dis-, meaning not, with ingenuous (from the Latin gen-, meaning born) which was originally used to distinguish free-born Romans from slaves, and later came to mean honest or straightforward. So disingenuous means dishonest. Ingenuous is less common now than disingenuous, but we still use it for someone who is sincere to the point of naiveté. A good synonym is insincere.

Definitions of disingenuous

adj not straightforward or candid; giving a false appearance of frankness

“"an ambitious, disingenuous, philistine, and hypocritical operator, who...exemplified...the most disagreeable traits of his time"- David Cannadine”
“a disingenuous excuse”
distorted, misrepresented, perverted, twisted
having an intended meaning altered or misrepresented
lacking sincerity
artless, ingenuous
characterized by an inability to mask your feelings; not devious
candid, heart-to-heart, open
openly straightforward and direct without reserve or secretiveness
without alteration or misrepresentation
open and genuine; not deceitful
naif, naive
marked by or showing unaffected simplicity and lack of guile or worldly experience
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