Why any next message in a thread is really nothing

In French, nobody is ne ... personne. Example: Il n’y a personne, there’s nobody there. But at the same time, personne does also mean person! This seems contradictory.

But it isn’t, because ne ... personne really means not a person, and so nobody, no-one.

The apparent strangeness is underlined by the fact that in the similar case ne ... pas, the ne part is often weakened in the spoken language:
Je ne sais pas, I don’t know, becomes Je sais pas, or even J’sais pas pronounced as [Zsep"A] or [Ssep"A].

If this also happens with personne, Je vois personne, the contradiction would become complete. I don’t know if French speakers really go that far.

Ne ... pas too can be explained from the meaning of the two words, as can many other cases:

Frenchdirect translationmeaning
ne ... pas not a step / pace not
ne ... que not that, but that only
ne ... personne not a person nobody
ne ... plus not more no more
ne ... rien not a thing / subject / object / issue nothing
ne ... jamais not ever never
ne ... point not a point not, no
ne ... mie not a crumb nothing any more
ne ... goutte not a drop nothing at all
ne ... guère not much hardly

Jamais without ne does really mean ever, sometime, at any time. Interestingly, Portuguese shares this apparently contradictory meaning, as the Porto Editora dictionary puts it: em tempo algum; nunca; alguma vez; nenhuma vez. This is spite of the fact that Portuguese doesn’t combine não with other words the way French does with ne. I wonder if there might have been French influence at work here.

The Portuguese word jamais originates from a combination of (yet, already) and mais (more).
The etymology of French jamais is similarly connected with déjà, considering

Le Petit Robert:
* jamais: o XIe; de ja, lat. jam « déjà », et mais, lat. magis
« déjà »
* déjà: o des ja 1265; de des (dé-) et a. fr. ja « tout de suite »,
lat. jam; cf. jadis, jamais
Le Littré:
* jamais: Jà, et mais dans le sens de plus ; comme qui dirait : jà
plus. Bourg. jaimoi ; nivern. jaimas ; provenç. jamais ; cat. jamay ;
espagn. jamas ; portug. jámais ; ital. giammai.
* déjà: Dès et jà ; bourguig. degy.

(The Portuguese word mais is not to be confused with mas, which does mean the same as French mais).

By the way, despite the identical spelling, the pronunciation is different, French [Za"me] or [Za"mE], Portuguese [Za"majS].

The French word rien derives from Latin rem, which is the singular accusative case of res, meaning thing, object, matter, affair, circumstance. The ablative case of that word is re, which was borrowed in English in the sense ‘in the matter of, on the subject of, about, concerning’. This is probably also the word Re: which appears in any subsequent message in an e-mail or usenet-news discussion thread. Because it is added only starting with the second message, people tend to mistake it for an abbreviation of "reply"; and so some programs have localised versions in which it is translated, e.g. to German Antw, Swedish Sv. In addition to being wrong (in my opinion anyway), this also often causes problems, because other programs don’t recognise these mistranslated Re:s, and so create long chains like
Antw: Re: Sv: Re:.

And because Re: really means nothing in French, everything but the first message in a discussion thread is in fact null and void.