Old English scield, scild "shield; protector, defense," literally "board," from Proto-Germanic *skelduz (source also of Old Norse skjöldr, Old Saxon skild, Middle Dutch scilt, Dutch schild, German Schild, Gothic skildus), from *skel- "divide, split, separate," from PIE root *skel- (1) "to cut." Perhaps the notion is of a flat piece of wood made by splitting a log. Shield volcano (1911) translates German Schildvulkan (1910). Plate tectonics sense is from 1906, translating Suess (1888).
Old English gescildan, from the root of shield (n.). Related: Shielded; shielding. Compare German scilden.