grating the teeth
齪 centers on the action of grinding or clenching the teeth, often with a sense of friction or tight pressure.
齪 combines 齒 (teeth) with 足 (foot), likely suggesting the action of teeth grinding together like feet pressing or moving. The exact historical development is uncertain.
The left side 齒 shows teeth, and the right side 足 looks like a foot pressing down. Imagine teeth grinding together with the pressure of a foot stepping on something hard.
For サク, picture grinding your teeth on a hard sack of grain: sack -> サク, and the rough texture makes you grate your teeth.