This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
匫 is a rare character whose historical development is uncertain. It visually combines the enclosure 匚 with the inner component 曶, but the exact origin and original meaning are unclear.
The outer box 匚 encloses a complex inner shape 曶, like a container holding something obscure. Since the meaning is unknown, picture it as a mysterious sealed box.
For コツ, imagine the box is full of old, dusty 骨 (こつ, bones): 骨 -> コツ, and the mysterious container holds ancient secrets.