This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 髛 is uncertain. It visually combines 高 and 尻, but the original semantic or phonetic role of these components is unclear.
With no clear meaning, a mnemonic is difficult. The components 高 (tall) and 尻 (buttocks) could suggest something elevated at the rear, but this is speculative.
For コウ, imagine a tall (高) person named Kou sitting on their buttocks (尻): Kou -> コウ.