Etymology
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knee (n.)

"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.

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knee (v.)
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.
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knee-deep (adj.)
c. 1400, "up to the knees," from knee (n.) + deep (adj.).
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knee-jerk (n.)
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.
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knee-high (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).
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kneepad (n.)
also knee-pad, 1858, from knee (n.) + pad (n.).
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kneecap (n.)
1650s, "a covering or protection for the knee," from knee (n.) + cap (n.). Meaning "bone in front of the knee joint" is from 1869; the verb in the underworld sense of "to shoot (someone) in the knee" as punishment is attested by 1975. Related: Kneecapped.
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*genu- (1)

Proto-Indo-European root meaning "knee; angle."

It forms all or part of: agonic; decagon; diagonal; geniculate; genuflect; genuflection; -gon; goniometer; heptagon; hexagon; knee; kneel; octagon; orthogonal; pentagon; polygon; trigonometry.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit janu, Avestan znum, Hittite genu "knee;" Greek gony "knee," gōnia "corner, angle;" Latin genu "knee;" Old English cneo, cneow "knee."

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geniculate (adj.)
"having knots or joints; bent like a knee," 1660s, from Latin geniculatus "having knots, knotted," from geniculum "little knee, knot on the stalk of a plant," diminutive of genu "knee" (from PIE root *genu- (1) "knee; angle"). Related: Geniculation (1610s).
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