younger of brothers, father's younger brother, younger brother of a husband
尗 centers on a junior male relative within a family: a younger brother, a father's younger brother, or a husband's younger brother. The common thread is a male relative of a younger generation or junior status.
The exact historical origin of 尗 is uncertain. It appears as a component in characters like 叔, where it likely contributes a meaning related to junior or younger status, but its independent development is not well documented.
Picture a small 小 person standing below 上 an older figure, like a younger brother looking up to an elder. The upper and lower positions reinforce the junior status.
For シュク, imagine a younger brother named Shook who always looks up to his older sibling: Shook -> シュク, and his name reminds you of the junior family role.