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.