This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
詥 combines 言 (speech) with 合 (join, fit). The exact historical development is uncertain, but it likely relates to words that harmonize or agree.
Words (言) that fit together (合) suggest harmony or agreement, but the kanji's meaning is too obscure for a reliable mnemonic.
For コウ, imagine a calm conversation where words fit together, and the sound of agreement is a soft 'koh': koh -> コウ.