Unduly means more than is reasonable. Unduly harsh, not unduly worried, unduly influenced.
It appears most in the negative, where not unduly worried is British understatement for quite worried. In law, undue influence is a defined ground for setting aside a will or a contract, and unduly lenient sentences can be referred for review.
1adverbundulyMore than is reasonable.
“The rent they were asking seemed unduly high for such a small flat.”
unduly is a degree.
Also accepted in games: unreasonably
Un- plus duly, from due, from Old French deu, owed, from Latin debere, to owe.
Middle English
duely
English
duly
English, today
unduly
Built from
From undue + -ly or un- + duly. duly comes from Middle English duely, dewly, dulyche, dueliche, duweliche (“rightly, properly”), equivalent to due + -ly.
Etymology from Wiktionary, CC BY-SA