According to
http://www.serv.net/~camel/wodehouse/jno.html:
...It was once the habit of men who wrote out legal documents and such for a living, to make abbreviations wherever possible. They were, after all, paid by the document, not by the letter. And so the name John, being a common name in England at this time, was commonly abbreviated something thusly:
| n o
_| ---
That is, with a big J, followed by a little n and a little o over a line. There being no easy way of representing this in type, when printing came to be done on a press there needed to be some way of representing this. As it happened, the way settled on was as "Jno." ...