The word 'unduly' is a relatively modern English formation, built upon the established adverb 'duly,' which means "in a proper, fitting, or owed manner." The prefix 'un-,' a common Germanic negation marker, is attached to reverse this meaning, creating the sense of "not in a proper manner." The core logic stems from the Latin concept of 'debere' (to owe), where something 'due' is what is rightfully expected or appropriate. Therefore, 'unduly' literally means acting outside the bounds of what is owed or proper, leading to its modern sense of excessiveness or inappropriateness.

Structure: un (not) + duly (properly) un: A prefix of Germanic origin (Old English, Old High German, Old Norse) meaning "not," used to form negatives. duly: An adverb from the adjective 'due,' originating from Old French 'deu' (owed, proper), from Latin 'debitus' (owed), past participle of 'debere' (to owe). The suffix '-ly' is an adverbial suffix from Old English '-lice.'