Origin: LL bisextilis, containing an intercalary day < bisextus < bis, twice (see bi-) + sextus, sixth: Feb. 24 (sixth day before the calends of March) was reckoned twice every fourth year
See bissextile in American Heritage Dictionary 4
Origin: Late Latin bissextīlis, containing an intercalary day
Origin: , from bisextus (diēs), bis sextus (diēs)
Origin: and Latin bisextum, intercalary day
Origin: : bis, twice; see bis
Origin: + sextus, sixth (because the sixth day before the Calends of March on February 24 occurred twice every leap year); see Sext.