adverb
Archaic interrogative adverb meaning 'how' or 'in what way'. Found in classical Japanese texts; not used in modern conversation.
「いかでかくは思ひたまふぞ」と古い物語にある。
In an old tale, it says, 'How can you think so?'
Derived from classical Japanese interrogative いか (how) + particle で (indicating means or manner). The exact historical development is uncertain.