Where does “achttien” come from?

achttien (Dutch) comes from Middle Dutch achtetien, from Old Dutch ahtotēn, from Proto-Germanic ahtōutehun, from Proto-Germanic ahtōu, from Proto-Indo-European oḱtṓw, from Proto-Indo-European aḱ- — sharp, pointed; sharp, pointed, edgy; sharp,...

achttien (Dutch): eighteen