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You're a friend of Tom's, aren't you?

Question: What does the 's in Tom's stand for?

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It doesn't stand for anything: it is simply the possessive clitic exactly as in Tom's friend.

There is an unobvious rule in English that though we say

my friend

and

Tom's friend

we don't normally say

*a friend of me

or

*a friend of Tom.

The idiomatic phrases are

a friend of mine

and

a friend of Tom's".

I hesitate to say the other forms are ungrammatical, but they are certainly not natural in current English. I think I can imagine a converation like

Who is that man over there? I don't know, I thought he was with you. I think he must be a friend of Tom.

Even in that context, "a friend of Tom's" is more likely, but I think "of Tom" is possible.

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  • I asked about this years ago on ELU citing this NGram as "evidence" that it's usually “friend of his”, but no possessive apostrophe with “friend of Peter”. And I don't remember anyone pointing out that my question was based on a false premise... Jun 25 '20 at 16:45
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    ...but seeing what you say here, I just concocted this NGram chart, which certainly backs up your assertion that the "apostrophe-less" form is less common with friend of Tom (personally, I'm happy with both; I don't think I've got any real preference). I guess I messed up that original NGram 'cos I didn't understand how to devise a valid query. Jun 25 '20 at 16:49
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It's another way of saying "You're one of Tom's friends, aren't you?"

The 's is the possessive.

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