An Etymological Dictionary of Astronomy and Astrophysics

1. The joint of the leg that allows for movement between the femur and tibia and is protected by the patella; the central area of the leg between the thigh and the lower leg.
   

2. Something resembling a bent knee, especially a rigid or braced angle between two framing members (Dictionary.com). → alpha element knee

Etymology (EN): M.E. kne; O.E. cneo, cneow “knee” (cognates: O.Norse kne, O.Sax. kneo, M.Du. cnie, Dutch knie, O.H.G. kniu, Ger. Knie; cf. Pers. zânu, as below.

Etymology (PE): Zânu “knee,” Mid.Pers. šnûg “knee;” Av. žnu- “knee;” cognates: Skt. jānu-, Hittite genu “knee;” Gk. gonu “knee,” gonia “corner, angle;” L. genu “knee;” O.E. cneo, as above; PIE *gnéwo-.