Origin of the term mil?

Can any of you kindly folks please tell me the origin
of the term 'mil' ,the one used in USA engineering 
and materials and such? Google, when presented with the
subject line of this post sans '?', gave, as the second
hit, a link to the explanation of mil gaya, apparently
an Indian swear word/phrase. This, of course, is 
completely useless to me at the moment, though may come
in handy to describe a Tandoor buffet I don't care for
(though I have yet to have that displeasure).

Wikipedia and several others possess a conspicuous absence
of a proper historical context for the origin of the term.

Apparently, it's also (intuitively) known as a 'thou'.

Help me a.f.c. , you're my only hope....

TTFN,
  Tarkin