synthetic fiber, 1959, American English, proprietary name, an arbitrary formation from expand + commercial suffix -ex.
Entries linking to spandex
expand v.
early 15c., "spread out, open out, spread flat, extend widely;" also transitive, "cause to grow larger;" from Anglo-French espaundre, Old French espandre "spread, spread out, be spilled," and directly from Latin expandere "to spread out, unfold, expand," from ex "out" (see ex-) + pandere "to spread, stretch" (from nasalized form of PIE root *pete- "to spread"). Related: Expanded; expanding.