The story
From earlier shell padde, equivalent to shell + pad (“toad”). Compare Saterland Frisian Skildpudde (“turtle, tortoise”), West Frisian skyldpod (“tortoise”), Middle Dutch schildpadde (Dutch schildpad, Afrikaans skilpad), Middle Low German schildpadde, Danish skildpadde. See also shield-toad.
Etymology from Wiktionary, CC BY-SA 4.0.