This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 甪 is uncertain. It may be a variant or simplification of an older form, but no clear origin is established.
The shape combines a top stroke 丿 with 用 (use). Since the meaning is unclear, picture a mysterious tool: a stroke added to 'use' that changes its purpose in an unknown way.
For ロク, imagine a lock on the mysterious tool: lock -> ロク, keeping its meaning sealed away.