'discreditably' originates from Latin, specifically the word 'discreditare', where 'dis-' meant 'not' and 'credere' meant 'to believe'.

'discredit' changed from Latin 'discreditare' through Old French/Anglo-French and Middle English forms (e.g. 'discreden', 'discrediten') and eventually became the modern English word 'discredit', from which the adjective 'discreditable' and adverb 'discreditably' were formed.

Initially, it meant 'to deprive of belief or credibility', but over time it evolved into its current sense of 'to harm reputation' and, as an adverb, 'in a manner that harms reputation' .