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.).
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.
"tall and sturdy, robust," originally applied to women, 1650s, from present participle of strap (v.), apparently in the sense of "to beat with a strap." Compare similar senses of whopping, spanking, bouncing and other present-participle adjectives of violent action expressing something large in size.
"joint between the principal bones of the leg," Old English cneo, cneow "knee," from Proto-Germanic *knewa- (source also of Old Norse kne, Old Saxon kneo, Old Frisian kni, Middle Dutch cnie, Dutch knie, Old High German kniu, German Knie, Gothic kniu), from PIE root *genu- (1) "knee; angle." For pronunciation, see kn-.
To be across (someone's) knee in reference to spanking is from 1866. Knee-breeches is from 1827; knee-pants is from 1858. Knee-slapper "funny joke" is from 1955.