From 杳(よう) (yō, “deep, dark, distant”) + 乎(こ) (ko, “state, appearance”).
杳(よう)乎(こ) • (yōko) †-tari, adnominal 杳(よう)乎(こ)たる (yōko taru), adverbial 杳(よう)乎(こ)と (yōko to) or 杳(よう)乎(こ)として (yōko to shite) (Classical Japanese)