what business; what matter
A somewhat formal or old-fashioned expression used to ask what someone's business is or why they have come. Often heard in samurai dramas or formal inquiries.
「何用で参られた？」と門番が尋ねた。
"What business brings you here?" asked the gatekeeper.
突然の来
At the sudden visit, he said suspiciously, "What is your business?"
Compound of 何 (nani, 'what') and 用 (yō, 'business, use'). The reading なによう is a straightforward combination of the two components.