fiftein
From Proto-Germanic *fimftehun. Compare Old Frisian fīftīne, Old English fīftīene, Old High German fimfzehan, Old Norse fimmtán, Gothic 𐍆𐌹𐌼𐍆𐍄𐌰𐌹𐌷𐌿𐌽 (fimftaihun).