abstruse, deep, profound
賾 centers on something that is not easily understood: deep, abstruse, or profound in a way that requires careful thought to grasp.
賾 is a rare character whose historical development is uncertain. The modern form visually contains 臣 and 責, but the exact origin of the left-side structure is unclear.
The left side looks like a minister (臣) peering into a deep, complex matter, while the right side 責 (responsibility) suggests the weight of understanding something profound. Together, 賾 represents the deep, abstruse nature of a subject that requires serious responsibility to comprehend.
For サク, imagine a scholar named Saku who spends his life studying abstruse texts: Saku -> サク, and his deep knowledge makes him the go-to person for profound questions.