late 14c., "not owing or payable," from un-
(1) "not" + pp. of due
. Formed on model of O.Fr. indeu, L. 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.