means A small handheld toy with weighted prongs that spin around a central bearing, briefly marketed as a focus aid and wildly popular as a fad.

from From the verb "spin," which comes from Old English "spinnan" — to draw out and twist fiber into thread, the same word that gave us spinning wheels and spinsters. The "-er" suffix simply names the thing that does the action, so a spinner is, plainly, a thing that spins. The fidget-toy sense is recent, attaching this ancient fiber-craft word to a piece of injection-molded plastic that never met a single thread.