button
鈕 centers on a small fastener used to join or close things: a button. The idea extends naturally to any small knob or catch that holds parts together.
鈕 combines 金 (metal) with 丑, which likely contributes the sound. The character originally referred to a metal button or knob.
The left side 金 suggests metal, and the right side 丑 looks like a small rounded shape with a stem. Picture a metal button on a coat, with the stem passing through the fabric.
For チュウ, imagine a metal button that makes a sharp 'chew' sound when you press it: chew -> チュウ, and the button clicks shut.