December

December comes from Middle English December, borrowed from Old French decembre, from Latin december, meaning 'tenth month.' That Latin word combines decem ("ten") with -ber, an adjectival suffix, since December was the tenth month in the Roman calendar.

The name was fixed when December sat tenth in the Roman year, but calendar reforms later shifted it to twelfth without ever changing its name.

December shares its root, Latin decem ("ten"), with the Proto-Italic *dekəm and ultimately Proto-Indo-European *déḱm̥, the ancient root for the number ten.