The word "December" is a Latin word that originally meant "tenth month." Though December is now the twelfth month in the modern calendar, it was the tenth month in the days of the Roman Empire. At this time, the calendar was only ten months long -- starting with March and ending with December.
The word "December" has two sub-parts in its original Latin. The root, "decem," means "ten." The suffix "ber" is thought to come from the Latin suffix "bris," which denotes the word as an adjective, changing its exact meaning from "ten" to "tenth."
December remained the tenth and final month of the Roman year until Nuna Pompilius, the second King of Rome who ruled around 700 BC, introduced two additional months into the year. These two months were Januarius and Februarius, later called January and February. They were inserted into the calendar prior to March, making December the twelfth month. However, its name and the names of the other months based on their position in the calendar stayed the same. Nuna did change the length of December, however. It had originally been 30 days long, and he shortened it to 29 days. Later, under the rule of Julius, December was lengthened to 31 days. However, it remained the twelfth month, regardless of its name's meaning.