feigned innocence; wolf in sheep's clothing
Describes a person who hides their true nature behind a gentle, innocent, or modest facade. Often used for someone who acts sweet but is actually cunning or calculating.
She's a wolf in sheep's clothing, so she's smiling even though she's actually angry.
That new guy is putting on an innocent act, but you never know what he's really thinking.
feigned ignorance; playing dumb
Specifically pretending not to know something, often to avoid responsibility or to manipulate a situation. Overlaps with the general 'feigned innocence' sense but emphasizes ignorance.
He played dumb and said he didn't know, even though he did.