the cuticle of the bamboo a broad-leaved bamboo
箬 refers to the outer skin or cuticle of bamboo, and by extension a type of broad-leaved bamboo. The meaning is narrow and botanical, with no clear abstract center beyond bamboo parts.
箬 combines 竹 (bamboo) with 若, which likely contributes a phonetic element. The exact historical development is uncertain.
The top 竹 is bamboo, and the bottom 若 looks like a young plant. Picture peeling the thin outer skin from a young bamboo shoot: that cuticle is 箬.
For ジャク, imagine a bamboo cutter making a sharp 'jak' sound as it slices through the tough cuticle: jak -> ジャク.