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.
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