All mouth and trousers is an excellent and evocative saying.
It is thought to have originated in the North of England. The remark is used to describe a man who brags vainly, i.e. he is all talk and clothing, nothing more – there is no substance to him or his claims (it may also imply that in fact there is very little to brag about inside the trousers).
Unfortunately, the saying is sometimes corrupted to include the word 'no'.
Why is there no 'no'?
1. All mouth and trousers came first and it makes sense.
2. All mouth and no trousers is a later, completely erroneous version. The first part still makes sense of course because it is unchanged. The second half with 'no' trousers makes no sense at all, especially as the braggart is the person least likely to wander around without trousers. He’d look ridiculous, which is the last thing he’d want.
3. There is another saying using the term 'flannel' which is the traditional material mens’ suits are made from:
‘it’s a load of flannel’
Again this means something has no substance. As with ‘no trousers’, it would be completely nonsensical to say ‘it’s a load of no flannel’.
The original saying in its correct form, I shall state it again for clarity, has many passionate adherents:
All Mouth and Trousers
There is an very extensive, amusing and informative blog dedicated to All Mouth And Trousers, find it by putting the saying into your search engine.