Why is the ninth month called September?
Old English undergan "undermine," from under + gan (see go). Cf. Middle Dutch ondergaen, Old High German untarkun, German untergehen, Danish undergaa. Sense of "submit to, endure" is attested from c.1300. Meaning "to pass through" (an alteration, etc.) is attested from 1630s. Related: Undergone; underwent.