'infidelitously' originates from Latin, specifically the word 'infidelitas,' where 'in-' meant 'not' and 'fidelitas' meant 'faithfulness.'

'infidelitas' transformed into the English word 'infidelity,' and eventually became the modern English word 'infidelitous' with the adverbial form 'infidelitously.'

Initially, it meant 'lack of faithfulness,' but over time it evolved into its current meaning of 'in a manner that is unfaithful or disloyal.'