Real Dictionary

etymon in American English
the earlier form of a word, as at an earlier period in the development of a language
noun: the linguistic form from which another form is historically derived, as the Latin cor “ heart,” which is the etymon of English cordial, or the Indo-European *ḱ(e)rd-, which is the etymon of Latin cor, Greek kardía, Russian serdtse, and English heart

etymon in British English
noun: a form of a word or morpheme, usually the earliest recorded form or a reconstructed form, from which another word or morpheme is derived: the etymon of English " ewe" is Indo-European "*owi"