How I love being right. The other day, two of my Northern coworkers were politely but skeptically listening to me telling them that “All mouth and no trousers” is a Southern corruption of the original Northern expression “All mouth and trousers”. I couldn’t remember where I had read it, so I went in search of an irrefutable source, and I was delighted to find a whole blog dedicated to this fine expression’s survival. I love the Internet. Apparently “All mouth and no trousers” and “All mouth and trousers” are the grey and red squirrels of idioms, the new corruption threatening the very existence of the original expression. I found an excellent description of the whole situation on The Telegraph website by Michael Quinion, author of the always excellent World Wide Words. Don’t miss the reference to “All fur coat and no knickers”, which James Ward taught me and which I love:

This strange expression comes from the north of England and is used, mainly by women in my experience, as a sharp-tongued and effective putdown of a certain kind of pushy, over-confident male. Proverbial expressions like this are notoriously hard to pin down: we have no idea exactly where it comes from nor when it first appeared, although it is recorded from the latter part of the 19th century onwards. However, we’re fairly sure that it is a pairing of “mouth”, meaning insolence or cheekiness, with “trousers”, a pushy sexual bravado. It’s a wonderful example of metonymy (“a container for the thing contained”).
The phrase seems to have become known, and surprisingly popular, among southern English writers in the last decades of the 20th century, perhaps as a result of the airing of a series of television comedies based in the North, such as the BBC’s Last of the Summer Wine. What is interesting about the saying from a folk etymological point of view is that its opaqueness has led its modern users to reinterpret it as “all mouth and no trousers”.
For example, an article in the Daily Record in 2002 quoted a Scottish politician as saying, “The First Minister is all mouth and no trousers”; a piece in the People newspaper described a pop group in the same terms; the Guardian in June 2002 said: “Bloody men. All mouth and no trousers.” It has reached the stage in which the older, non-negative form is in great danger of vanishing, though Australia and New Zealand seem to be staying with it (when they use it at all, which isn’t often).
Metropolitan writers are trying here to make sense of something obscure that they have not often heard in its native surroundings, and are getting it muddled. They confuse it with other put-downs that are conventionally phrased with a negative, such as “all talk and no action” or “all fur coat and no knickers”. To have no trousers on is not only embarrassing, the argument seems to go, but is a state in which one is not ready for action (outside the bedroom, that is).
It’s a pity it should be changing through ignorance. It’s a lovely phrase, as effective a snub as anyone could want – all the better for being slightly obscure – and it’s one that ought to be preserved pristine.

I just have one tiny quibble with this analysis: I don’t think the negative implies a change in the use of the metonymy. To me, “trousers” also represents the sexual organ in “no trousers” and is used in exactly the same way, not to indicate that the person is embarrassed or incapacitated by a lack of “real” trousers, but that he isn’t man enough to act. Am I wrong?
Gratuitous photo of the beautiful North of England, taken on Lindisfarne, Northumberland.