Lewis Short
bĭ-sextus | bissextus, i | bĭsextum (noun M.n) : (erroneously ; v. bis
* Fin.), , m., sc. dies (, , n., Censor. 20; Amm. 26, 1, 7) [bis], an intercalary day; so called, since the 24th of February = VI. Cal. Mart., was doubled: bisextus est post annos quattuor unus dies adjectus,Isid. Orig. 6, 17, 25; Dig. 50, 16, 98; 4, 4, 3, § 3; Macr. S. 1, 14; Aug. Trin. 4; Isid. Orig. 6, 17, 25-27.
Charlton T. Lewis, Charles Short, A Latin Dictionary