This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 箰 is uncertain. It visually combines 筍 (bamboo shoot) with 子 (child), but the original semantic or phonetic role of these components is unclear.
The top 筍 suggests a bamboo shoot, and the bottom 子 is a child. Picture a child holding a bamboo shoot, but the overall meaning remains unclear.
For シュン, imagine a child soon (シュン) picking a bamboo shoot: soon -> シュン, and the child reaches for the shoot.