Why is the ninth month called September?
early 13c., "offense, crime, sin," later (late 13c.) "failure, failure to act," from Old French defaute (12c.) "fault, defect, failure, culpability, lack, privation," from Vulgar Latin *defallita "a deficiency or failure," past participle of *defallere, from Latin de- "away" (see de-) + fallere "to deceive, to cheat; to put wrong, to lead astray, cause to be mistaken; to escape notice of, be concealed from" (see fail (v.)). The financial sense is first recorded 1858; the computing sense is from 1966.
late 14c., "be lacking, be missing," also "become weak," from default (n.). Related: Defaulted; defaulting.
Failure to pay a debt when it is due.