'unconstitutionally' originates from English formation of the negative prefix 'un-' + 'constitutional' + the adverbial suffix '-ly', where 'un-' meant 'not' and '-ly' formed an adverb.

'constitutional' comes from Middle English/Old French 'constitution' (Old French constitucion) and ultimately from Latin 'constitutio' (from 'constituere'), and 'unconstitutional' was formed in English by adding the prefix 'un-' to 'constitutional'; the adverb 'unconstitutionally' was formed by adding '-ly' to that adjective.

Initially, the elements referred to 'that which is established or set up' (from Latin), and the combined modern word has come to mean 'not in accordance with the constitution'; the basic legal sense has been retained and specialized.