sincerity, admonish, warn, prohibit, truth, fidelity
誠 centers on inner truthfulness and its outward expression: sincerity, fidelity, and honest truth. The admonish/warn/prohibit senses are less common and may reflect a separate usage where truthful words serve as a warning.
誠 combines 言 (speech, words) with 成, which likely contributes the sound and the idea of completion or becoming. Together they suggest words that are fully realized or true.
Words (言) that are fully formed and complete (成) are sincere and true. Picture speaking only when your thoughts are fully formed, so every word is honest.
For セイ, imagine a sincere person saying 'say' with complete honesty: say -> セイ, and their words are always true.
truth; reality; fact
sincere; honest; faithful
really; truly; very; extremely; exceedingly; sincerely
loyalty; sincerity; allegiance; fidelity; integrity
sincerity; good faith
insincerity; dishonesty; untruthfulness; bad faith
sincerity
sincerity; devotion
insincerity; dishonesty; untruthfulness; bad faith
principle of good faith
better shoddy and sincere than clever and deceptive
to be loyal to; to serve; to offer allegiance
to pledge allegiance; to swear an oath of allegiance; to take an oath of allegiance
loyalty; faithfulness
earnestness
sincerity can move heaven
Shiseikan Hall University
thank you very much
in all sincerity; truthfully; honestly
to do with sincerity