Meaning

noun a whimsically eccentric person

noun a pitch with reverse spin that curves toward the side of the plate from which it was thrown

adjective foolish; totally unsound

originFrom screw + ball. Compare typologically German Spinner (akin to spin). Also compare weirdo and weird, from Old English wyrd (“fate”), ultimately derived from Proto-Indo-European *wert- (“to turn, to spin”).

Etymology from Wiktionary, CC BY-SA 3.0