CC stands for carbon copy.
Basically, it comes from an old fashioned way of making a copy of something. In the past (long before email), if you wanted to make a copy of something such as a letter, you had to use special carbon copy paper.
You could write the letter on normal paper, but you needed to place this carbon copy paper beneath it while you were writing. That would automatically make another copy of your writing. You can still see carbon paper in some things like science lab notebooks, where you have to handwrite your data and can have an automatic carbon copy when you finish writing.
Now that almost everything is digital, we still use the abbreviation CC to refer to copying someone on an email who was not originally intended as the direct recipient. In essence, it is creating a “carbon copy” of the email for them to see.