Why is the ninth month called September?
1620s, "wrinkled" (of skin, etc.), past participle adjective from corrugate. Meaning "bent into curves or folds" (of iron, cardboard, etc., for elasticity and strength) is from 1853.
1620s; implied earlier as a past participle adjective (early 15c.), from Latin corrugatus, past participle of corrugare "to wrinkle very much," from com-, intensive prefix (see com-), + rugare "to wrinkle," of unknown origin.