Old coin and very thin?

I answered this yesterday, asking for the measurements. Now that you've provided that, here is what I think you have:

I believe you have a 1902 1/3 farthing from Great Britain. 1902 was the only year this denomination was issued during the short reign of Edward VII. The specifications of the coin is 11 mm, but wear and and your measurement being not 100% precise can account for a tiny 1 mm difference. The coin has suffered a split planchet at some point in its life, with the reverse part that shows the date being lost. This would account for the piece you have being very thin, and also your being able to see the 'mirror image' wording from the front.

It's possible the split occurred when the coin was holed, perhaps to wear on a chain, which was a common practice in Victorian times and into the 20th century. Maybe it was done purposely. But the combination of the missing piece and the holes makes it worth very little, unfortunately. These coins in higher grades are quite scarce and sought by collectors, but yours would be useful only as a hole filler or a novelty coin.

I _think_ it's British, but the size is throwing me, as well as the legend.

A British (or British territory) coin from the reign of Edward VII would have something like the following

EDWARDUS VII D.G. BRITT OMN REX FD IND IMP which is Latin, and somewhat abreviated, for his title.

But the smallest example I can find from that time is a 1/3 farthing coin that was used in Malta. Even that is nearly half again as large as what you're describing.

Without some good pictures to see what you really have, that's about the best I can do.

Edit: Just saw the previous answer, and I think he's got it right.