1727, "to strike forcefully with the open hand, especially on the buttocks," possibly imitative of the sound of spanking. Related: Spanked; spanking. The noun is from 1785.
Entries linking to spank
spanking adj.
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.
spanking n.
"act of striking with the open hand," especially as a punishment administered to children, 1854, verbal noun from spank (v.).