Why 3rd party not 2nd?

[NorkyPies]

(Before getting to my main point, I'm intrigued to see that Pedants' Corner is classified under 'Other', rather than 'Fun and Games', by MN.)

'Third party' - why do we speak of a 'third party' when often, or even usually, the party concerned is the second. This is prompted by the Gareth Williams case currently in the news, where there is mystery about whether he could have locked himself inside a sports bag, or someone else (referred to as 'third party') was involved.

Doesn't it come from language, eg first, second, third person, etc? Second person is directly addressing someone ie 'you'

Because it refers to a grammatical third party, not a numerical third party.

So "I" am first person

"You" are second person.

And "he" or "she" is third person.

So if you are talking about someone who is not yourself or the person you are speaking top, then you are talking about a "third person".

Yes, I wondered about the grammatical third person; but then why 'third party' and not 'third person'? It looks like some sort of crossover between grammarians' and lawyers' usage.

Third party can plural, third person implies only one other person - whereas ther could be many?

OK titchy - 'third person, possibly plural' would be unwieldy. Perhaps 'third party' is the acceptable compromise.