'pretrimmed' originates from the prefix 'pre-' (from Latin 'prae') meaning 'before' combined with the verb 'trim' (from Old English 'trymman' / Middle English 'trimmen').

'trim' changed from Old English 'trymman' (meaning to make firm or prepare) to Middle English 'trimmen' and eventually the modern English 'trim'; the prefix 'pre-' was attached in modern English to form 'pretrim' and then 'pretrimmed'.

Initially, 'trim' meant 'to make firm or prepare'; over time it shifted to the sense 'to cut, shape, or make neat'; adding 'pre-' yielded the sense 'trimmed beforehand' as in 'pretrimmed'.