'eco-activist' originates from Modern English as a compound of the prefix 'eco-' (short for 'ecology') and 'activist' (from 'activism'), where 'eco-' referred to 'environment/house' and 'activist' meant 'one who takes action'.

'eco-' comes from 'ecology', which ultimately traces back to Greek 'oikos' meaning 'house' or 'environment'; 'activist' developed from the adjective 'active' (Latin 'activus') and the noun-forming suffix '-ist' in English, combining to form 'activist' in the 19th–20th centuries; these parts were joined in modern English to form the compound 'eco-activist'.

Initially, 'activist' meant simply 'a person who is politically or socially active'; over time the compound 'eco-activist' evolved to mean specifically 'a person who takes action to protect the environment'.