This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
毣 combines 羽 (feathers) and 毛 (hair/fur), suggesting fine, soft hair or feathers. The exact historical development is uncertain.
Feathers 羽 above hair 毛: imagine a bird with soft downy feathers that feel like fine hair. This kanji suggests something soft and fluffy.
For ボク, picture a soft feather boa (ボア) that feels like fine hair: boa -> ボ, and the soft texture cues ボク.