This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 斣 is uncertain. It visually combines 蜀 and 斗, but the original semantic or phonetic role of these components is unclear.
Since the meaning is unclear, focus on the shape: 蜀 on the left and 斗 on the right. Imagine a measuring cup (斗) being used to compare something from the 蜀 region, but treat the meaning as unknown.
For シュク, imagine a rare book (書, しょ) being measured with a 斗 cup, but the reading is シュク, so think of a 'shook' (invented word) cup shaking as it measures: shook -> シュク.