'demagnetize' originates from Latin/modern English formation, specifically from the prefix 'de-' (a Latin prefix meaning 'remove' or 'reverse') combined with 'magnetize' (from 'magnet' + '-ize').

'demagnetize' changed from the verb 'magnetize' (from 'magnet' < Middle English/Old French) with the addition of the prefix 'de-' in modern English to form 'demagnetize'; the adjective/past form 'demagnetized' derives directly from that verb form.

Initially, 'magnet' referred to the lodestone from Magnesia; with the suffixes and prefix it came to mean 'to remove magnetic properties,' and this core meaning has remained stable in modern usage for 'demagnetize' and 'demagnetized'.