sincere, faithful, surname
諶 centers on sincerity and faithfulness in speech or character; the surname sense is a specific application of that same idea of trustworthiness.
諶 combines 言 (speech) with 甚, which likely contributes the sound and may reinforce the idea of deep or sincere words. The exact historical development of 甚 in this compound is uncertain.
言 (words) beside 甚 (extreme) suggests words that are extremely honest and faithful: sincere speech.
For シン, imagine a sincere person named Shin who always speaks faithfully: Shin -> シン, and his words are 諶.