play dumb

To pretend that one has no or little knowledge (of something); to act ignorant or uninformed (about something). I don't think I'm supposed to know that Rick is getting fired, so I'm just going to play dumb the next time I see him. A: "Did Tammy ask you about my date with Steve?" B: "Yeah, but I played dumb about it. I just couldn't handle more of her drama today."