What would you add to a Martini to make it a "dirty"?

The Martini is as classic as cocktails get. But not all Martinis are the same. The Dirty Martini is believed to have originated in 1901 when New York bartender John O’Connor found inspiration in the famous olive garnish. First made by muddling the olive into the drink and later by adding a splash of olive brine, the Dirty Martini took decades to reach a wide fan base. It eventually found favor among drinkers. In the modern day, the cocktail is equally loved and reviled.

Source: Liquor.com