'truly' originates from Old English, specifically the word 'trēowlīce', where 'trēowe' meant 'faithful' and '-līce' was an adverbial suffix.

'trēowlīce' transformed into the Middle English word 'trewely', and eventually became the modern English word 'truly'.

Initially, it meant 'in a faithful manner', but over time it evolved into its current meaning of 'in a truthful or genuine manner'.