This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
訫 combines 言 (speech) and 心 (heart/mind), suggesting sincerity or speaking from the heart, but its exact historical development is uncertain.
Words 言 coming from the heart 心 suggest sincere communication, but since the meaning is uncertain, treat this as a tentative image.
For シン, imagine speaking sincerely from the heart: the word 'sincere' starts with 'sin', which cues シン.
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