honest speech
讍 centers on honest, forthright speech. It is a rare character with a narrow meaning, so treat it as a specific term for speaking truthfully without deception.
讍 combines 言 (speech) with 噩, which likely contributes the sound and may relate to seriousness or awe, reinforcing the idea of solemn, honest speech. The exact historical development is uncertain.
Words 言 paired with the solemn, multi-mouthed 噩 suggest speech that is serious and truthful, as if many voices confirm the honesty.
For ガク, imagine a person speaking honestly at a gaku (がく, music) performance, where only truthful praise is allowed: gaku -> ガク.