As with many of my favorites, it's a Prohibition-era gin cocktail that has recently enjoyed a renaissance.  It's equal parts Luxardo maraschino cherry liqueur, green chartreuse, gin, and fresh lime juice.  The maraschino liqueur and chartreuse both give a pungent, punchy smell and flavor.
The Last Word also has a mesmerizing pale green color that almost goes neon in some lights.  I like to garnish mine with a Luxardo cherry or Amarena cherry.