We often hear these two expressions used interchangeably, probably because of their paronymy, and we therefore tend to confuse them. In the same way, the terms "attention" and "intention" are sometimes confused when an expression like "à l'intention" is used for "à l'attention".
Definition of quick
The expression "in a jiffy" means that something is done quicklyAt least as quickly as you can turn your hand. The same meaning is generally given to tour de main. It should be noted, however, that tournemain is rarely used other than in the expression "en un tournemain", whereas "tour de main" is used with variations: un bon tour de main.
Émile Littré states in his Dictionary of the French language that in his day, at the end of the 19th century, "today we say en un tour de main", then adds "when we want to express something that is done quickly, we should say: en un tournemain, and not: en un tour de main". And Littré goes on to say that tour de main refers to "an act of skill performed with the hand".
In its 8th edition, the Académie française adds that the expression has become outdated and condemns it, so to speak. Sleight of hand or sleight of hand?
Definition of tour de main
In his Dictionary of medicine, surgery, pharmacy, veterinary medicine and related sciences... (1884), Littré gives the following definition:
"Fam, In a jiffy, In as little time as it takes to turn your hand.
We took the case off in a jiffy. I would have done it in a jiffy. Some say and write, "In a jiffy".
But it also states " Turn of hand, turn of subtlety, turn of skill. This cup-player does some amazing tricks.
Whether we look at etymology or usage, we can see that the expressions tend to be used for each other. And it is also understandable that the vocabulary has a certain tendency to simplify, if only because we are unaware of their status as paronyms.
I'll mess you up in a jiffy!
Petit Larousse and Grand Robert keep their dictionaries up to date with the latest French expressions. They regularly introduce new words, as if they were afraid that literature wouldn't appropriate them. Or that the little people would appropriate them before they did? Like them, the Académie's dictionary seems to endorse certain usages, but at a more modest pace. Whether it's the Trésor de la langue française or the Encyclopédie, our lexicographers compare research and points of view from all eras and, above all, bear witness to the use of words by those who could write, whatever their level of mastery of the language of their time... Quarrels between linguists and grammarians will only help to retrace the history of words and enrich encyclopaedic knowledge, a treasure trove. Let us see behind the rule an attempt to simplify an overly complex code, to establish a glossary or lexicon for the French-speaking community, but let us not see in it the compulsion to take for granted what the first dictionary that comes along claims! When one expression is replaced by another, the French language is impoverished. When we neglect to do our job as lexicographers, we contradict ourselves...
Quick turnaround, dexterity and skill
Whatever people say, the two expressions continue to coexist, but any lover of the French language will recognise that it's easy to understand that speed of execution and address are confused, if only because it takes a good hand to execute a gesture quickly. In the 3rd edition of its dictionary (1740), the Académie also stated that tricks of the trade are "tricks of subtlety and skill that are performed with the hands", and she adds that definition of tour de main in the 1937 edition Fig. and fam, Having the knack, To have the way of doing things, the skillpractice.
Contradicting yourself is still saying something, but it's not convincing. In short, it's usage that will prove the Académie and the dictionaries right, or not! In the meantime, feel free to distinguish between the two expressions and use each of them!