The adverb toot sweet means straight away, immediately.

Humorously after the English words toot and sweet, it represents an anglicised pronunciation of the synonymous French adverb tout de suite.

Before the First World War, it was only used in representations of French speech. For example, an article titled Galloglossia, published in Sharpe’s London Magazine for March 1848, denounces the way “John Bull must discard his honest, straightforward, manly language, and adopt in its stead the mincing and distorted speech of his [French] neighbours”:

In the days of stage coaches we heard in the coffee-room of an inn much frequented by those creeping vehicles, a good stout representative of our insular greatness shout “Garsong!” and a clumsy, greasy waiter respond “Toot sweet, Mounseer!”

It was during World War One that toot sweet began to gain wider currency, because it was used by the English-speaking soldiers fighting on the Western Front in order to communicate with the locals. The earliest instance that I have found is from Tommy Atkins as the darling of all France: loved and respected because he is such a good fellow, by the British journalist, author and translator John Nathan Raphael (1868-1917), published in the Sunday Pictorial (the former name of the Sunday Mirror – London) of 25^th April 1915:

Mr. Atkins, rather more than six feet of him, was standing at the bottom of a hill, near a village in France. In his right hand, uplifted, was a little milk can. Down near the left putteed knee of him were two little French boys, grinning and gazing up into his face. “Look here, you little beggars,” Mr. Atkins was saying, “do you parly Frongsy at all?” “Mais oui, M’sieu’! Oui, M’sieu’!” said the two boys excitedly. “Right-o!” said Mr. Atkins. “Then lookey voo here. D’ye see that farmhouse on the top of the hill?” The boys followed the pointing finger and nodded. Mr. Atkins lifted two fingers of his left hand. “Ung, doo, one, two, tuppence,” he said. “You scoot up that hill, d’you see, to that little farm at the top, and, speakin’ the language, you ses, ses you, ‘The British Army’s waitin’ at the bottom of the hill,’ you ses, ‘for two-penn’orth of doo lay!’” The boys took the milk can and the twopence. Mr. Atkins put his two hands to his mouth and shouted after them, “Toot sweet!”

There have been variants, such as toot and sweet and toot de sweet. For instance, in Sister Barbara, a story published in the Evening Telegraph (Dundee, Scotland) of 10^th June 1919, Cherry Plummer, a sailor, says to an office clerk:

“You’re head chap here, ain’t you? Well, when I strike this place again, you don’t give me no talk at all. I go bang in to the head Panjandrum—like as if I was come to buy the business almost. I walks right in to see Mr Thingumabob, toot de sweet, all on the knocker, P.D.Q.! Right in every time. See? That’s me! Call me ‘sir’! I fancy it!”

The phrase the tooter the sweeter is an alteration of the sooner the better usually intensifying a preceding use of toot sweet.
—Cf. also, on the same pattern, the more firma, the less terra, from terra firma.

The phrase the tooter the sweeter is first recorded in Punch, or the London Charivari of 5^th December 1917 in the caption to this cartoon:

THE NEW LANGUAGE.

Tommy (to inquisitive French children). “Nah, then, alley* toot sweet, an’ the tooter the sweeter!”

(* alley, for French allez!, go away!)

I have found the variant the sweeter the tooter in a letter from an American soldier published in The Bisbee Daily Review (Bisbee, Arizona) of 21^st April 1918:

“Tout de Suite,” which is pronounced toot sweet for short, and which in plain United States means “do it now,” is one of the first expressions we have to learn and it works overtime. An exasperated American with a limited French vocabulary, put it thusly to some local workmen: “Here, you, do that toot sweet, and the sweeter the tooter.”

The adverb toot sweet gave rise, in WWI military slang, to the noun toot-sweeter, denoting a cannon, in allusion to the rapidity of approach of the shells. The Daily Mail (Hull, Yorkshire) of 31^st December 1921 published a review of an article by Edgar Preston, What the Soldier said (National Review – December 1921), containing the following:

Many and various were the nicknames bestowed on the guns of all calibres. There were “Grandmother” or “Grandmama” (British 15-inch gun), “Grannie” (9.2 gun), “Minnie” (German trench mortar, or Minenwerfer), “Pom-pom” (French .75. the famous “Soixante-quinze”). “Big Boys” stood for any type of big gun, of which “Billy Wells” was one. Other picturesque names for cannon which I have noted are: “Coal-box,” “Coughing Clara,” “Crump,” “Grasshopper,” “Lazy Eliza,” “Pip-Squeak,” “Whistling Percy,” “Toot-sweeter,” “Dirt.”