This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
牿 combines 牜 (cow) with 告 (announce). The exact historical development is uncertain, but it likely relates to cattle or enclosures.
The left side 牜 suggests a cow, and the right side 告 looks like a mouth over earth. Imagine a cow being led into a pen with a gate announced by a sign.
For コク, picture a cow being locked in a pen: the sound 'lock' is close to コク, and the cow is secured.