inquire, question, numerous
詵 centers on the act of asking or seeking information: inquiring or questioning. The sense of 'numerous' likely extends from the idea of many questions or a crowd of people asking.
詵 combines 言 (speech) with 先, which likely contributes the sound and may suggest going forward or ahead, perhaps related to questioning or a crowd moving forward. The exact historical development is uncertain.
Words (言) going ahead (先) like a stream of questions: when you inquire, your words lead the way. The 'numerous' meaning can be remembered as many words pouring out.
For シン, imagine a shin-high stack of inquiry forms: shin -> シン, and you have to question each one.