E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Leap-year. We add a day to February in leap-year, but the Romans counted the 24th of February twice. Now, the 24th of February was called by them dies bissextus (sexto calendas Martias), the sextile or sixth day before March 1st; and this day being reckoned twice (bis) in leap-year, was called annus bissextus.