The story
Coined by Thomas Henry Huxley in 1872 as ento- + plastron. Superseded the earlier entosternum, a nominalised form of the French adjective entosternal, coined by Étienne Geoffroy Saint-Hilaire in 1818. The prefix is from Ancient Greek ἐντός (entós, “inner”), in reference to its position as the unpaired bone at the centre of the plastron.
Etymology from Wiktionary, CC BY-SA 4.0.