to chew
齚 centers on the action of chewing, specifically using the teeth to grind or crush food.
齚 combines 齒 (teeth) with 乍, which likely contributes a phonetic element. The character is a phono-semantic compound where the teeth radical indicates the meaning related to chewing.
The left side is 齒 (teeth), and the right side 乍 looks like a person with an open mouth. Picture teeth clamping down on food as the jaw works: 齚 is to chew.
For サク, imagine chewing on a sack lunch: sack -> サク, and your teeth are grinding the food inside.