'pre-' + 'treated' originates from the prefix 'pre-' (from Latin 'prae', meaning 'before') combined with 'treat', which comes from Old French 'traiter' from Latin 'tractare' (to handle, to manage).

'treat' comes via Old French 'traiter' (from Latin 'tractare' / 'trahere') and was adopted into Middle English as 'treten'/'treaten'; the productive English prefix 'pre-' (from Latin 'prae') was attached to create 'pre-treat' in modern English to mean 'treat beforehand', and the past/participle form became 'pre-treated'.

Initially 'treat' meant 'to handle, manage, or negotiate'; over time it gained senses including 'apply a process to' (e.g., apply a chemical or preparation), and 'pre-treat' specifically came to mean 'to apply such a process before the main treatment or procedure'.