Wikia

The Plumber's Hat is Vanity Item which is part of the Plumber's Outfit. It is an exceedingly rare drop from Fire Imps (¹/₅₀₀) in the Underworld. The rest of the Plumber's Outfit can be bought from the Clothier.

TriviaEdit

This hat is based on the hat of video game character Mario, from the well known Nintendo franchise.
A way to farm the hat is to fill up any possible spawn areas with any type of blocks (ash block preferred but does not affect the spawn rate) such as ceilings, obsidian towers, and position yourself in the middle of the screen. Stocked up of at least 2 full stacks of the Battle Potion and the Water Candle and make a 2x2 shelter. Set up a minimum of 2 dart traps, one on your left and right (A Blood Moon doesn't hurt because it also increases spawn rates).
On the console version, the Plumber's Hat that is dropped may be blue. (Confirmed on both XBOX360 and PS3)