'flame-retardant' is a compound formed from English 'flame' (from Old English 'flām', meaning 'flame') and 'retardant', from French 'retardant', the present participle of 'retarder', ultimately from Latin 'retardare' where 're-' meant 'back' and 'tardare' meant 'to delay'.

'retardant' entered English from French 'retardant' (present participle of 'retarder'); the compound 'flame-retardant' arose in technical and safety contexts in the 20th century to describe substances or treatments that delay burning.

Originally related to the general sense 'to delay' (delay or slow). Over time, in this compound it narrowed to the specific meaning 'to delay or inhibit combustion', i.e., a substance or property that slows or prevents fire.