'fossil-free' originates from Modern English, formed as a compound of 'fossil' and 'free'; 'fossil' ultimately comes from Latin 'fossilis' meaning 'dug up', and 'free' comes from Old English 'frēo' meaning 'not under the power of'.

'fossil' came into English via Late Latin 'fossilis' ("dug up") and was used from the 17th century onward to name remains and, later, fuels derived from such remains; 'free' evolved from Old English 'frēo' into modern English 'free'. The compound 'fossil-free' is a recent Modern English formation used in environmental and energy policy language (20th–21st centuries).

Initially, 'fossil' meant 'dug up (from the ground)' and later applied to substances formed from ancient organic remains; 'free' has long meant 'not subject to' or 'without'. Together, the compound evolved to mean 'without the use of fossil fuels' in contemporary environmental discourse.