Quantova

In the years 1673/1674, Madame de Sévigné's and her friends started to refer to Madame de Montespan as "Quantova" or "Quanto".
Why such a nickname and what was the meaning of it ?

In that time, the snobish must among the French elite was to read and speak Italian, and the nickname was referring to the quote Madame de Sévigné once (a few years later) wrote in its entirety : "La Pedina del Re quanto va".

Pedina and Re belong to the Italian chess vocabulary, and they appear in a Italian proverb which says "Alla fin del gioco tanto va nel sacco il re quanto la pedina" which translates "After the game, the king and pawn go into the same box". Doubtlessly, that was the origin of the quote, but when the proverb was referring to death, madame de Sévigné was playing with the other meaning of "pedina".
In Italian, "pedina" means "pawn" but it was also the word used for an illegitimate wife. In that light, the "king and pedina in the same box at the end" had a very different meaning.

So, "Quantova" was a private joke on Madame de Montespan being the king's mistress and on how long she would be able to hold the position.