The colloquial British-English phrase to be all mouth and (no) trousers means to tend to talk boastfully without any intention of acting on one’s words—synonym: to be all hat and no cattle—cf. also gas and gaiters, blue sky and hot air and big girl’s blouse.

The original form of this phrase is to be all mouth and trousers (i.e. without the negative determiner no), mouth and trousers probably denoting verbal and sexual arrogance respectively.

It is first recorded in the following dialogue from Two for the River (Hamish Hamilton – London, 1961), by the British novelist and essayist Leslie Poles Hartley (1895-1972):

‘It’s not a bad life. Most men are all mouth and trousers—well, I like the trousers best, if you see what I mean.’
‘You mean without the trousers.’
‘Yes, I suppose I do.’

The earliest occurrence that I have found is from It’s a long, long way to council harmony, by Stuart Gilles, published in The Birmingham Post (Birmingham, Warwickshire) of Tuesday 10^th January 1967:

Jack Judge, the nearest thing Oldbury has had to a bard, once wrote a song called Tipperary.
The second line says: “It’s a long way to go.” Perhaps, if he had written it today instead of 50 years ago, he might have named it Warley.
For, as council meeting follows council meeting, it is very clear that Warley still has a very long way to go.
This month’s meeting followed the form which, by now, has been accepted by some almost as traditional.
Basically, a third was council business—dealt with as efficiently as Warley knows how—and the other two-thirds were what might be called elsewhere “all mouth and trousers.”

The earliest instance that I have found of the form with the negative determiner no is from the column Keith Waterhouse¹ on Thursday, titled that day The Young Pretender, in the Daily Mirror (London) of Thursday 18^th July 1974:

When I heard the news from Cyprus, the years rolled back and I was standing at the bar of the Ledra Palace Hotel, Nicosia, in the mid-nineteen fifties.
It was the regular evening session when journalists met to compare notes, and as usual there was this slightly preposterous youth on the fringe of the group.
He was a talkative but flippant young man much given to sniggering, and if you’d told me then that he would be the next President but one of Cyprus, I would have firmly removed the glass from your hand and advised you to go away and sleep it off.
Not that young Nicos Sampson² wasn’t suspected—along with every second waiter and taxi-driver—of being up to his ears in Eoka³ activities. It got noticed, in time, that he was always just that fraction ahead of everybody else with news of another killing on Murder Mile. No-one was very surprised when the security men finally tapped him on the shoulder.
But the consensus view of the older hands was that, politically, he was all mouth and no trousers. It was a view I quickly came to share myself.

¹ Keith Waterhouse (1929-2009), British novelist, newspaper columnist and screenwriter
² Nikos Sampson (1934-2001), Greek-Cypriot journalist and militant nationalist
³ EOKA, acronym from Ethniki Organosis Kyprion Agoniston (National Organisation of Cypriot Fighters): a Greek-Cypriot nationalist organisation that fought for the end of British rule in Cyprus, for the island’s self-determination and for eventual union with Greece

The use of the negative determiner no in the phrase might have arisen from analogy with idioms of similar meaning such as all style and no substance and all talk and no action—cf. also all dressed up with nowhere to go.

In this respect, it is interesting that in the following from Steve Grant’s review of David Jones⁴’s production of Bertolt Brecht⁵’s Baal, published in The Observer (London) of Sunday 19^th August 1979, to be all mouth and no trousers appears among expressions built on the pattern to be all —— and no ——:

The problem is that while attempting to relate the play to our own neuroses the production comes across as ‘arty’ in the worst sense. It is all effect and no core, all nightmare and no vision, all mouth and no trousers.

⁴ David Jones (1934-2008), English stage, television and film director
⁵ Bertolt Brecht (1898-1956), German playwright, producer and poet

There have been earlier occurrences of expressions built on the pattern to be all mouth and no ——.

For instance, the following is from an article about the American boxer Muhammad Ali (Cassius Marcellus Clay – 1942-2016), published in the Florence Morning News (Florence, South Carolina) of Thursday 7^th March 1963:

Just in case anyone thinks young Cassius is all mouth and no brains, a check on the Madison Square Garden boxoffice could be very enlightening.
It appears the Garden is headed for its first boxing sellout since middleweight champion Jake LaMotta⁶ beat Italy’s Tiberio Miltri⁷ on July 12, 1950.

⁶ Giacobbe ‘Jake’ LaMotta (1922-2017), American boxer
⁷ Tiberio Miltri (1926-2001), Italian boxer

And, in her column Your Money’s Worth, titled that day What Poverty Law Isn’t, in The Arizona Daily Star (Tucson, Arizona) of Wednesday 4^th November 1964, Sylvia Porter quoted one expression built on the pattern to be all mouth and no ——, as well as one on the pattern to be all —— and no ——:

It also is not, as its critics have claimed, a “hastily assembled package of odd legislative retreads and rejects, interspersed with a sprinkling of old, unsuccessful experiments” and it will not turn out to be “all promise and no progress . . . all mouth and no muscle.”