bêtise
The word ‘bêtise’ means ‘a foolish, ill-timed remark of action; a piece of folly’. The origins of the word are rooted in French, where it translates to ‘stupidity’.
The Norman Conquest of England in 1066 saw French become the official language of England. As it made its way into the affairs of public office and the judiciary, French became widespread amongst the nobility. Although English eventually replaced French as the official language in 1362, members of the English elite continued to use loanwords from the latter as a signifier of their prestige. Indeed, it was not uncommon for upper-class English children to be taught French.
Although Isabel is American, her use of French denotes her assimilation into English upper-middle-class society.
Singh, I. (2005) The History of English, London: Hodder Arnold.
Braugh, A. C & Cable, T. (2002) A History of the English Language, London: Pearson Education.