The story
From sail + duck (“canvas-like material, rag”). Cognate with Saterland Frisian Saildouk, West Frisian seildoek, Dutch zeildoek, German Low German Seildook, German Segeltuch, Danish sejldug, Swedish segelduk, Icelandic segldúkur.
Etymology from Wiktionary, CC BY-SA 4.0.