play dumb

verb3

1. •
   (informal) To pretend to be dumb, either:
2. •
   (informal) To feign muteness.

   • I played dumb 'cause I knew if I talked at all, being simple and guileless, you all would twist me up and have the whole thing in a jiffy. — 1904
3. •
   (informal) To feign stupidity or ignorance of some knowledge, usually in order to avoid responsibility or gain some advantage.

   • Valchek: Kid, careers have been launched on a helluvalot less. Just... shut up and play dumb. Hauk: I can do that, no problem. — 2006