The term "fire plug" dates from the early 1800s, when water mains were made from wood. The fire department (usually volunteers) would head out to the fire, dig up the cobbles down to the main, then chop into the main so that they could place the hoses from their pumpers. When finished fighting the fire, they'd seal the main with -- you guessed it -- a "fire plug." The next time there was a fire in the neighborhood, they'd dig up the plug and not have to cut into the main.

Many of the departments were volunteer, and large cities had competing volunteer departments who would charge after-the-fact for putting the fire out. (The nastier ones would charge before-the-fact, while you were not in a negotiating mood.) If there was a plug near the fire, the volunteers would send out their biggest goons to guard it from the other volunteer departments. The goons were called "plug-uglies," which is where that term came from.