'matter-of-factly' originates from English, specifically formed from the adjective 'matter-of-fact' with the adverbial suffix '-ly', where 'matter' ultimately comes via Old French 'matiere' from Latin 'materia' meaning 'material/substance', and 'fact' comes from Latin 'factum' meaning 'a thing done'.

'matter-of-factly' developed from the phrase 'matter of fact' (early modern English), which became the hyphenated adjective 'matter-of-fact' and was later converted to the adverb 'matter-of-factly' by adding '-ly' in modern English.

Initially the phrase described something 'relating to or concerned with facts' (as opposed to feelings); over time the derived adverb came to mean 'in a straightforward, unemotional manner' used to describe how something is said or done.