knee (n.) Look up knee at
"joint between the principal bones of the leg," Old English cneo, cneow "knee," from Proto-Germanic *knewam (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" (source also of Sanskrit janu, Avestan znum, Hittite genu "knee;" Greek gony "knee," gonia "corner, angle;" Latin genu "knee"). 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.
knee (v.) Look up knee at
early 13c., "to bend the knee, kneel," from Old English cneowian, from cneow (see knee (n.)). The meaning "to strike with the knee" is first recorded 1892. Related: Kneed; kneeing.