The story
From red + nugget + galaxy. From the highly red colouration of the galaxies, and high density and compactness, like a dense gold nugget.
Etymology from Wiktionary, CC BY-SA 4.0.
red nugget galaxy
Today it means: A type of passive, massive and compact elliptical galaxy, that has been little changed over many billions of years, having little internal activity, little to no interaction with other galaxies, little to no new star formation in billions of years. The galaxies typically are many times more massive, and many times smaller, than our Milky Way Galaxy. The redness grows as stars age, which as a population, become redder with time. Full definition →