What does the word million originally mean?

It is Italian - milione, a big thousand, built from mille plus an augmentative ending. It entered English in the fourteenth century, and before that there was no word for the quantity in most European languages because nobody had any use for one.

Above a million the naming system splits and has caused real confusion. Britain historically used the long scale, where a billion was a million million; the United States used the short scale, where it is a thousand million. Britain officially switched in 1974, but older British texts and much of continental Europe still use the long scale, so a billion in a French or German document is a thousand times larger than in an English one.

The scale of the difference between them is worth feeling rather than reading. A million seconds is about eleven and a half days. A billion seconds is nearly thirty-two years. The words differ by three letters and the quantities by a factor of a thousand, which is a large part of why numbers in public argument are so easy to misuse.