This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
矠 combines 矛 (halberd) with 昔 (old times). The exact historical development is uncertain, but it likely relates to an ancient weapon or tool.
The left side is a halberd 矛, and the right side is old times 昔. Imagine an ancient halberd from long ago, but its exact use is unclear.
For サク, picture an old halberd stuck in a sack: sack -> サク, and the ancient weapon is stored away.