Late Middle English; earliest use found in Mirour of Mans Saluacioune. From post-classical Latin primogenitus (adjective and noun) first-born (Vetus Latina, Vulgate), probably from classical Latin prīmus + genitus, past participle of gignere to beget, after Hellenistic Greek πρωτότοκος first-born (Septuagint, New Testament; the post-classical Latin word probably thus shows the connecting vowel -o- rather than the expected -i- after Hellenistic Greek πρωτό- in πρωτότοκος), or perhaps from classical Latin prīmō (adverb) first + genitus.