The story
From the resemblance to the vegetable. From being a galaxy having a greenish tinge, and being small and round like a pea.
Etymology from Wiktionary, CC BY-SA 4.0.
green pea galaxy
Today it means: A type of galaxy, found in the local universe, that are active, undergoing high rates of star formation, which provides a green tinge, and are compact, visually appearing akin to peas. These active galaxies have strong oxygen emission lines and no active galactic nucleus (“AGN”), similar to small galaxies in the distant early universe, but unlike those in the nearby contemporary universe. Full definition →