'plant-eaters' originates from Modern English, specifically the compound 'plant-eater', where 'plant' meant 'a young shoot or anything planted' (from Latin 'planta') and 'eat' meant 'to take food into the mouth and swallow', with the agentive suffix '-er' meaning 'one who performs an action'.

'plant' came into English via Old French 'plante' from Latin 'planta' (meaning 'sprout, cutting'); 'eat' comes from Old English 'etan'. The compound 'plant-eater' was formed in Modern English by combining these elements; the plural form 'plant-eaters' follows regular English pluralization.

Initially it referred straightforwardly to 'animals that eat plants' and over time has remained largely the same in meaning, often used informally alongside the technical term 'herbivore'.