Why is the ninth month called September?
late 14c., "spiritual, incorporeal," from Medieval Latin immaterialis "not consisting of matter, spiritual," from assimilated form of in- "not, opposite of" (see in- (1)) + Late Latin materialis (see material). Secondary sense of "unimportant" is first recorded 1690s from material in its 16c. sense of "important." Related: Immaterially.