John is not only a very common English name, but actually an English word and part of English expressions. Let me tell you about some of the very interesting meanings of John.
First, we have the noun john, which is used (although not in formal English) both for the clients of a prostitute (the British equivalent is punter) and for the toilet (for example, to sit on the john). Then, of course, there is John Doe, a name used to either refer to a male that is supposed to remain anonymous or to an average man (the corresponding name for females is Jane Doe), see here. Finally, I have recently learned that a Dear John letter is a letter written from a woman to her geographically separated partner to inform him about the end of their relationship. The expression has its origin in letters written by women to oversea soldiers, see here.