Why is the ninth month called September?
early 15c., from Anglo-French remayn-, Old French remain-, stressed stem of remanoir "stay, dwell, remain; be left; hold out," from Latin remanere "to remain, to stay behind; be left behind; endure, abide, last" (cf. Spanish remaner, Italian rimanere), from re- "back" (see re-) + manere "to stay, remain" (see mansion). Related: Remained; remaining.
"those left over or surviving," mid-15c., from Middle French remain, back-formation from Old French remanoir, remaindre, or else formed in Middle English from remain (v.). But the more usual noun in English has been remainder except in remains, euphemism for "corpse," attested from c.1700, from mortal remains.