kneecap (n.)youdaoicibaDictYouDict[kneecap 词源字典]
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.[kneecap etymology, kneecap origin, 英语词源]