This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 勀 is uncertain. It visually combines 克 and 力, but the original semantic and phonetic roles are unclear.
The kanji has no clear meaning, so a mnemonic is not applicable.
For コク, imagine a rare, ancient artifact called a 'koku' that looks like the kanji 勀, with 克 and 力 combined. The sound 'koku' is the only clue to its mysterious purpose.