An "etto" means exactly 100 grams.

"Etto" is an abbreviation of the word "ettogrammo", but you will hardly ever see the full form of the word in a street market. The exact English translation of "ettogrammo" is "hectogram" and, just as a kilogram is 1,000 grams, so a hectogram is 100 grams. This is a little less than four ounces and is a convenient unit of weight for selling olives, herbs and so on.

More Info: en.wikipedia.org