To the Romans, an ordinary soldier was miles (pronounced meeless), from mille, which literally means “a thousand” but in the figurative sense means “a great many, a horde.”
The new recruit, to distinguish him from the seasoned campaigner, was a tiro (plural, tirones).
In Medieval Latin, the words were often spelled tyro, tyrones, and it is with this spelling that the word is most often used in English, although the spelling tiro, which may be preferable on an etymological basis, is sometimes used.
The English plural, incidentally, is tyros or tiros, and the word has come to have the extended meaning of “a novice in any field, a greenhorn.”
