Looked it up in the Dutch database for given names:
Joost——> root “Jodocus”, also “Justin”.
And I was wrong with the “typical Dutch” as the entry for the root “Jodocus” is:
Name of Breton origin. Judoc, from *Judacos “experienced in battle”. Later associated with Greeek ioodokos, “cntains arrows”. See also Joyce and Josse (French form of the name). Saiint name: son of Breton king turned hermite in Brahic (ca. 644), died in ca. 699, festivity day 13 Dec.