Tout de suite is French for at once, or immediately. It is often written as tout suite, which is a misspelling: the de belongs in it. First World War soldiers anglicised it as toot sweet, recorded from 1917.
What’s the origin of the phrase ‘Tout de suite’?
At once, or right away, is the literal translation of this French phrase. This is very often written incorrectly as tout suite. The English have never been comfortable with adopting French terms into English and, compared with Latin phrases say, there aren’t as many as we might expect considering how close the two countries are geographically. This ambivalence, and the fact that most English can’t actually speak French or even make a decent stab at pronouncing French terms, is the source of the attempts to poke fun at the French by deliberately mispronouncing those French phrases that have made it into English - in this case as toot sweet. This teasing dates back to the First World War; for example:
Arthur Empey, in Over the top, 1917 - “‘Toots Sweet.’ Tommy’s French for ‘hurry up’, ‘look smart’.
Punch, 1917 - Tommy (to inquisitive French children): “Nah, then, alley toot sweet, an’ the tooter the sweeter.”
Both of those quotations are from 1917, and 1917 is also the date Etymonline gives for toot sweet, which it records as American English, “representing U.S. soldiers’ mangled adaptation of French tout de suite”. That sits alongside the Tommy of the quotations rather than against him: the title page of Empey’s book describes its author as “an American soldier who went”, a machine gunner serving in France, and the glossary his definition comes from is printed at the back of the same book as Tommy’s Dictionary of the Trenches. Empey’s entry also runs on past the part quoted above, into the estaminet: the phrase was “generally used in a French estaminet when Tommy only has a couple of minutes in which to drink his beer”.
The 1968 film Chitty Chitty Bang Bang included a song called Toot Sweets. This led to the marketing of confectionery with the same name, in the form of sugar whistles.
The phrase excuse my French takes this a step further in implying that French is inherently linked with profanity. More recently, this phenomenon has been made fun of in the many comic mistranslations of French phrases put into the mouth of Del Boy (David Jason) in John Sullivan’s comedy Only Fools and Horses: mange tout - used by Del Boy to mean ‘my pleasure’ and creme de la menthe - ‘the very best’.
On the French side of it there is nothing in doubt. The Cambridge Dictionary’s French to English entry lists tout de suite as an adverb and translates it as at once, immediately. The de is not padding either: Etymonline glosses de suite, inside the phrase, as “in sequence”. The two spellings that go wrong in English, tout suite and toute suite, are the two that leave the de out.