What does this dictionary entry mean?

Facing a Scrabble challenge: the word si.

The entry in Merriam-Webster Collegiate, 11th ed., says

n [It] (1728).

which I read as “noun, Italian, first recorded instance in 1728.”

Then follows this:

superscript 2 followed by TI in small caps, with no space in between, as 2TI—double-checked with a magnifying glass.

What does this 2TI (superscript 2) mean?

It’s significant that it says “noun,” which I think means it can’t be “yes,” since I believe yes and no are classed as adverbs.

That’s all there is. No definition otherwise. Nothing.

I need somebody who knows Italian and/or dictionary notation to help me interpret this.

And yes, I consulted all the front pages but did not find an explanation of the small caps used like this.