The story
Of onomatopoeic origin. Compare Saterland Frisian klatskje (“to smack, slap”), West Frisian kletse, kletskje, Dutch kletsen (“to smack, slap, clash”), German Low German klattsen, klatsken (“to smack, splash”), German klatschen (“to clap, smack, slap”) and Klatsch (“a clapping sound; the din resulting from two or more things colliding”), Danish klaske (“to clash, splatter”).
Etymology from Wiktionary, CC BY-SA 4.0.