know in detail
讅 centers on thorough, detailed knowledge: knowing something in full detail, not just superficially.
讅 combines 言 (speech, words) with 審, which likely contributes both sound and the idea of examining or judging in detail. The character suggests knowing something thoroughly through careful inquiry or discussion.
Words (言) and careful examination (審) come together: imagine a scholar questioning every detail until they know the subject completely.
For シン, picture a detective examining a scene in detail, thinking "I've seen (シン) everything now." The word "seen" cues シン while the detective's thorough inspection matches the kanji's meaning.