'treater' originates from Modern English, formed as an agent noun from the verb 'treat', which ultimately comes from Old French 'traitier' (or 'traiter'), itself from Latin 'tractare', where the root 'tract-' (from 'trahere') meant 'to pull, draw or handle'.

'treat' entered Middle English from Old French 'traitier/traiter' (and related medieval forms) and from Latin 'tractare'; the agent suffix '-er' was added in English to form 'treater'.

Initially the root sense was related to 'pulling, drawing, handling' (Latin); over time the verb 'treat' evolved to mean 'deal with, handle, give medical care, entertain, or process', and 'treater' came to mean 'one who treats' in these senses.