septuaginta.

From Proto-Indo-European *septm̥̄ḱomt, from earlier *septḿ̥-dḱomt (“seven-ten”). According to De Vaan, the -ā- was introduced under the influence of other cardinal numbers, such as sexāgintā (“sixty”). De Vaan suggests that *septḿ̥dḱomt evolved into Proto-Italic *septmā-, a development also affirmed by Sihler. Both De Vaan and Sihler propose that early *septmā- developed into *septumā-, with Sihler postulating an intermediary phase *septomāginta. According to Sihler, this form may have emerged due to the contamination of terms such *oktowāginta, a possible pre-form of octōgintā. Regardless, this form *septumā- may have then transformed into *septuwā-, whence septuāgintā. Cognate with septem and Ancient Greek ἑβδομήκοντα (hebdomḗkonta).