Where dose the phrase "Tom,Dick or Harry" come from?

[gerry]

I have heard this phrase being used a number of times "They were letting in any old Tom,Dick or Harry".

Any idea why it's Tom,Dick or Harry?

[Quizmonster]

Here are two possibilities:-
a. Tom, "Richard" - to pacify the site censor - and Harry Dunsden were brothers who became notorious highwaymen in Oxfordshire in the late 1700s. All three died at the hands of the law in one way or another and their exploits became the stuff of legends. Two of them were hung in chains after their execution and these chains were, apparently, still to be seen as recently as the 1930s.
b. I've read that the phrase is an anglicised version of a Cornish one, "an tamm, diek or'n arja", which means "the clod of a farmer at the plough". In other words, 'just about anybody' as the modern phrase implies. (I have never been convinced that this was not a Cornish Mickey-take, though!)