Amazon - Two Items Received Instead Of...
Shopping & Style4 mins ago
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For more on marking an answer as the "Best Answer", please visit our FAQ.The Oxford English Dictionary says the etymology of 'plug-ugly' is obscure, but it quotes a reference from Bartlett's American Dictionary, published in 1860, to the effect that it did - as Clanad says - refer to ruffians. However, Bartlett adds that it was earlier used by fire companies.
As I understand it, what we in Britain call a fire-hydrant is called a fire-plug in the USA. I suspect, therefore, that the reference may well be to the squat, shapeless, featureless characteristics of that device.