(as) mute as a fish

Totally silent; not saying a word. I can't believe you sat there, mute as a fish, as the boss criticized me in the middle of the meeting. Thanks for nothing! Oh, so now you're all as mute as a fish, how convenient. Is no one really going to tell me who's responsible for this prank? The boy was wide-eyed and mute as a fish as he watched the barrel explode into flames.

render (one) mute

To cause one to stop speaking or become unable to speak. News of Allison's death rendered me completely mute, and I sank to my knees. This couldn't be happening. When I let the curse word slip, it rendered my mom mute for a few seconds. Then the yelling started.