Etymology of _John_ 'Prostitute's Client'

[Shapiro, Fred]

I have been asked about why the word _john_ is used to denote a prostitute's client.  It seems obvious to me that the name John, because of its commonness, became a generic term for men, perhaps with the implication that prostitute's clients don't give their real names.  But can anyone point me to any  discussions, in dictionaries or scholarly books or articles, of the derivation of this usage?  I have looked at the OED, HDAS, Green, and Dalzell already.

Fred Shapiro

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