This chiefly North American slang term for a ruffian or thug has a claim to fame in that it’s the only word in the English language that contains the letter string ugug (apart, that is, from pug-ugly, which was an attempt at turning plug-ugly into a word that made more sense). It may also be one of the more obscure of slang terms, though this is perhaps too large a claim to withstand much enquiry. But its obscure origins and odd form have generated more tries at explaining it than almost any other.
What we do know is that the word was first applied to one of the notorious gangs that terrorised the big cities of the eastern US in the years before the Civil War. The infamous Fourth of July riot in New York in 1857 is said to have been between the Bowery Boys, joined by the American Guards, and an alliance of the Plug-Uglies and the Dead Rabbits.
The Plug-Uglies originated in Baltimore and — like other gangs of the time — were politically aware and active. An early reference to them was in a report from Washington just a month before the New York riot:
A gang of organized desperate rowdies, some fifty in number, called the “Plug Uglies”, arrived here this morning from Baltimore, for the purpose of defeating the Democratic ticket.
New York Daily Times, 2 Jun. 1857. They were supported by two local gangs, the Rip-Raps and the Clunkers. The Plug-Uglies managed to acquire a cannon from the Navy Yard, but were unable to fire it. They were eventually dispersed by the US Marines.
Various attempts have been made to explain plug from its various senses. One tale is that gang members were ugly because they had been plugged — punched in the face. Plug is recorded at the time for a homely person, which might make plug-ugly a reduplicated compound. It has been argued that the name derives from competing Baltimore fire-fighter companies who became combative around fire-plugs. In his Oxford Etymologist blog, Anatoly Liberman suggests that as plug in its various senses is of Dutch origin and as it means a subordinate or servant in the current dialect of Groningen, it might have had the meaning of a wicked underling. An early report claimed that the Plug-Uglies got their name because of the plug hats they wore, stuffed with paper and forced down over their ears as improvised protective headgear. Yet another story reads like a tale told to a naive foreigner:
“Plug-Uglies” ... Several years ago I was in Baltimore, where the class of rowdies who originated this euphonious name abounded, and was told it was derived from a short spike fastened in the toe of their boots, with which they kicked their opponents in a dense crowd, or, as they elegantly expressed it, “plugged them ugly”.
The Times, 4 Nov. 1876.
Whichever story is correct (I favour the one about the plug hats), the name had reached Belfast by 1856 and the events of 1857 were widely reported in the British press, though plug-ugly didn’t enter the local vocabulary. By the middle 1860s, the term had lost its capital letters in North America and had become generic for a ruffianly and rowdy gang member. I suspect that the works of one writer in particular made it more familiar to British readers:
“Why, say, suppose a plug-ugly sasshays [sic] up to you on the street to take a crack at your pearl stick-pin, do you reckon he’s going to drop you a postal card first?
The Coming of Bill, by P G Wodehouse, 1920.