köken: Internationalism. Borrowed from Russian дека́брь (dekábrʹ); itself from Old East Slavic декѧбрь (dekębrĭ), from Old Church Slavonic декѧбрь (dekębrĭ), from Byzantine Greek Δεκέμβριος (Dekémbrios). Ultimately from Latin december, from decem + Latin -ber, from -bris, an adjectival suffix.
köken: Borrowed from Latin (mensis) december (19th century). Replaced older decemvrie, which came from the same source but through a Byzantine Greek (cf. Δεκέμβριος (Dekémvrios)) and Slavic intermediary.
By haplology from earlier *decumo-mēmbris, analogically to septem : September, from earlier *decumo-mēnsris "of the tenth month", from the ordinal stem decem (“ten”) + *mēnsris, from mens- (“month”) + -ris. In the Roman calendar, the year began with Mārtius (“March”), and December was the tenth month of the year.