Why is the ninth month called September?
1550s, "abnormality of growth," from Late Latin monstrositas "strangeness," from Latin monstrosus, a collateral form of monstruosus (cf. French monstruosité); see monster. Earlier form was monstruosity (c.1400). Sense of "quality of being monstrous" is first recorded 1650s. Meaning "a monster" is attested from 1640s.