I think it means something along the lines of “ That is false!”, however, I’m not entirely sure.
put on hold as off-topic by Toto, Stéphane Gimenez♦ Dec 7 at 15:07
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- "Please look up the meaning of words or expressions in a dictionary first. If you did so and found nothing satisfactory, mention that in your question. Do give context for where you heard or saw the word." – Toto, Stéphane Gimenez
If we trace this particular use of "mais" to its definition in the TLFi we end up at II B 2;
II [Mais s'emploie en tête d'un énoncé en réaction à une situation dont le locuteur refuse telle ou telle conséquence ou telle ou telle conclusion qu'on pourrait en tirer.]
["Mais" is used at the beginning of a statement as a reaction to a situation of which the locutor refuses to accept a particular consequence or a particular conclusion that can be drawn from it.]
B [L'énoncé introd. par mais est mis en relation avec la situation extra-linguistique et non avec un énoncé qui le précède.]
[The statement introduced by "mais" is connected with the extra-linguistic situation instead of a preceding statement.]
2 [[Mais] introduit ce qui n'est pas prévisible dans la situation où l'on est; indique que la situation ne permet pas de comprendre ce qui a lieu et à quoi l'énoncé introduit.]
[[mais] introduces something that is not predictable in the present situation; indicates that it's not possible to understand what is happening and what the enonciation leads to.]
I think that to the definition can be added that in some cases, as the present one (Mais c'est faux!), there is an element of surprise that can be communicated by "mais", in particular through intonation, although the essential part of it will have to be placed on the "c'est faux".
The translator "DeepL" gives "But that is not true!" and "But it's not true!" and that could also be translated as "But it's false!" this is a basic way to render "mais", however not the only one.
Essentially for the reasons given I think that fundamentally there is no better translation for "mais" in the present context than "why" although, unfortunately it is old fashioned.
- Why, it's wrong!
A reason that makes this word seem preferable is its long vowel, which lends itself more readily to the superimposition of an intonation that will connote various degrees of surprise or possibly irritation and other feelings.
A word that doesn't have the defects of "but" and "why" is "wow"; however, the register is somewhat informal and it is used to express great surprise.
- Wow! It's false!
Perhaps the ideal word as a general purpose word for average levels of surprise and not at all colloquial would be "say". "well" and "Oh well" would do also.
- Say, it's false!
- Well, it's false! _ Oh Well, it's false!