'dissociate' originates from Latin, specifically the Late Latin word 'dissociare'/'dissociatus', where the prefix 'dis-' meant 'apart' and the root 'soci-' (from 'socius') meant 'ally' or 'companion'.

'dissociate' passed into French as 'dissocier' and came into English from Late Latin/French forms (e.g. Late Latin 'dissociatus' → French 'dissocier') and eventually became the modern English word 'dissociate'.

Initially it meant 'to separate from alliance or companionship', and over time it broadened to mean 'to separate or disconnect' in general contexts (mental, chemical, social).