Hacker News Reader

In particular, compare: “mon sieur” vs. “mes sieurs”.

Indeed

English words of French origin are normally considered formal. Back when the French invaded England the rich spoke French, while the poor spoke Anglo Saxon. The words mutated and changed but words of French origin are now formal because of this.

As a French, it feels weird to learn something about my language on an English site :|. I don't remember seeing it written in French. Or I saw it, understood the meaning and forgot.

You’ve probably heard it without noticing - eg “Mesdames et messieurs” (ladies and gentlemen) is still in common use.

That's because it's largely unused nowadays, the current abbreviation being MM. (which doubles the M. – not Mr! – abbreviation for monsieur).