Waterloo was a town in Belgium. It was the nearest settlement to the site of Napoléon Bonaparte's defeat at the Battle of Waterloo. (VOY: "The Thaw")
The battle was mentioned in the song "I Am the Very Model of a Modern Major-General" by Gilbert and Sullivan. (ST: "Q&A")
Although the Clown referred to Napoleon "after Waterloo", he most likely was referring to the battle. However, since the battle was named after the town of the same name, the two references are combined here as one.