The story
Borrowed from Japanese 北(ほっ)海(かい)道(どう) (Hokkaidō, “North Sea Circuit”), from Middle Chinese 北海 (pək-hʌi˧˥, “Northern sea”) + 道 (dáu, “Circuit”, a province-like administrative unit from Tang dynasty China).
Etymology from Wiktionary, CC BY-SA 4.0.