JohnyoudaoicibaDictYouDict[John 词源字典]
masc. proper name, mid-12c., from Medieval Latin Johannes, from Late Latin Joannes, from Greek Ioannes, from Hebrew Yohanan (longer form y'hohanan) literally "Jehovah has favored," from hanan "he was gracious."

As the name of John the Baptist and John the Evangelist, it was one of the most common Christian given names, and in England by early 14c. it rivaled William in popularity. The Old French form was Jean, but in England its variants Johan, Jehan yielded Jan, Jen (also compare surname Jensen). Welsh form was Ieuan (see Evan), but Ioan was adopted for the Welsh Authorized Version of the Bible, hence frequency of Jones as a Welsh surname.[John etymology, John origin, 英语词源]
john (n.)youdaoicibaDictYouDict
"toilet," 1932, probably from jakes, used for "toilet" since 15c. Meaning "prostitute's customer" is from 1911, probably from the common, and thus anonymous, name by which they identified themselves. Meaning "policeman" is 1858, from shortening of johndarm, jocular anglicization of gendarme.