The adverb of 'unjustifiable' — it describes HOW something was done, saying that it was done in a way no reason can excuse. Because it is an adverb, it attaches to a verb ('the police unjustifiably held him for two days') or, very often, to another adjective, where it works rather like 'far too... and there is no excuse for it': unjustifiably high prices, an unjustifiably long wait, unjustifiably harsh treatment. That second use is the one you will meet most. It is a formal word, at home in reports, journalism and legal writing, and it carries the same weight of blame as 'unjustifiable'. In everyday speech people would just say 'for no good reason at all'.

無正當理由地、說不過去地（副詞，常接在形容詞前，如 unjustifiably high 貴得毫無道理）