8 ft lance
矟 centers on a long pole weapon: specifically an eight-foot lance used in historical contexts.
矟 combines 矛 (halberd/lance) with 肖, which likely contributes a phonetic element. The character specifically denotes a long lance.
The left side 矛 is a halberd or spear, and the right side 肖 looks like a small moon over a body, suggesting a long, slender weapon. Picture an eight-foot lance with a blade that gleams like a crescent moon.
For サク, imagine a soldier thrusting the lance into a sack of grain: sack -> サク, and the long lance pierces it easily.