One of the most peculiar things about languages is that over a period of time their grammar and semantics become surrogate to public opinion. Hence, it doesn't matter what your English teacher thought about that composition you wrote way back in the sixth standard. Chances are, if she revisits it today, she might award you a few more marks than she did last time.
Now changes can be orthogonal, in that a word or phrase may come to mean something completely unrelated to its original meaning. "Presently" being a case in point. When I was a young kid, it used to mean, "soon", or if you were into Wodehouse, "anon". Now it seems, it is generally accepted to mean "at present" or "currently". Then there are some whose meanings do a volte-face and become their own antonym. Which brings us to the topic under discussion, "the full Monty".
Popular opinion, courtesy of a movie of the same name, interprets this phrase to mean completely uncovered, or naked, or without embellishment. Here's the story behind its origin:
The most often-repeated derivation is from the tailoring business of Sir Montague Burton. A complete three-piece suit, i.e. one with a waistcoat, for a wedding etc, would be the Full Monty. There is plausible hearsay evidence from staff who worked in Burton's shops who confirm that customers were familiar with the term and often asked for 'the full monty' by name.
So, the next time you get invited to a formal do, you might want to go the full monty, without fear of being arrested on grounds of indecent exposure ;-).
Source: http://www.phrases.org.uk/ and my meandering experience.