1660s, "very big or fine," later (especially of horses) "moving at a lively pace" (1738), of uncertain origin; perhaps from a Scandinavian source (OED compares Danish spanke "to strut"). Probably also related to spanker "something striking" (for size, etc.), 1751; as a kind of sail from 1794.

"act of striking with the open hand," especially as a punishment administered to children, 1854, verbal noun from spank, v..