to bite
齰 centers on the action of biting, specifically using the teeth to cut or grip something.
齰 combines 齒 (tooth) with 昔, which likely provides the sound. The character is used specifically for the action of biting.
The left side 齒 shows a tooth, and the right side 昔 looks like something old or past. Imagine an old, tough piece of food that you have to bite down on hard with your teeth.
For サク, picture biting into a sack of crunchy snacks: sack -> サク, and your teeth sink in.