mid-14c., from knock (v.). As an engine noise, from 1899.

"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.

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.

Old English cnocian (West Saxon cnucian), "to pound, beat; knock (on a door)," likely of imitative origin. Figurative meaning "deprecate, put down" is from 1892. Related: Knocked; knocking. Of engines from 1869. To knock back (a drink) "swallow quickly or at a gulp" is from 1931. Many phrases are in reference to the auctioneer's hammer, for example knock down (v.) "dispose of (something) at auction" (1760).

1774, from knock (v.) + knee (n.).

1660s, "arouse by knocking at the door," from knock (v.) + up (adv.). However it is little used in this sense in American English, where the phrase means "get a woman pregnant" (1813, in a New Jersey context), possibly ultimately from knock in a sense "to copulate with" (1590s; compare slang knocking-shop "brothel," 1860). To knock up also meant "to knock unconscious" in early 19c.

Knocked up in the United States, amongst females, the phrase is equivalent to being enciente, so that Englishmen often unconsciously commit themselves when amongst our Yankee cousins. [John Camden Hotten, "The Slang Dictionary," London, 1860]

Also "to exhaust, overcome or make ill with fatigue" (1737).

patellar reflex, a neurological phenomenon discovered and named in 1876; see knee (n.) + jerk (n.1) in the medical sense. The figurative use appeared soon after the phrase was coined.

c. 1400, "up to the knees," from knee (n.) + deep (adj.).

1743, from knee (n.) + high (adj.). Phrase knee-high to a grasshopper first recorded 1851 (earliest form was knee-high to a toad, 1814).

also knockdown, 1680s, from the verbal phrase knock down, attested from mid-15c. in the sense "fell to the ground;" see knock (v.) + down (adv.). As a noun from 1809. Phrase knock-down, drag-out is from 1827.