1387, "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 1398. Sense of "unjustifiable" is attested from 1399 (implied in
unduly). Meaning "excessive" is first recorded 1684.