'fire-retardantly' originates from English, specifically formed from the adjective 'fire-retardant' (itself from 'fire' + 'retardant') and the adverbial suffix '-ly'; 'retardant' ultimately traces to French and Latin roots meaning 'to delay'.

'fire-retardantly' developed in Modern English by adding the suffix '-ly' to 'fire-retardant'; 'retardant' came into English via Middle French 'retarder' and from Latin 'retardare' (from 're-' + 'tardare'), which in turn relates to Latin 'tardus' meaning 'slow'.

Initially the underlying root carried the sense 'to delay' or 'to make slow', but over time the compound came to mean 'having the property of slowing or resisting combustion', and the adverb now means 'in a manner that resists or slows fire'.