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late 14c., "not owing or payable," from un- (1) "not" + past participle of due (adj.). Formed on model of Old French indeu, Latin indebitus. Meaning "not appropriate, unseasonable" is recorded from late 14c. Sense of "unjustifiable" is attested from c.1400 (implied in unduly). Meaning "excessive" is first recorded 1680s.