'assistency' originates from English, specifically formed from the verb 'assist' + the noun-forming suffix '-ency', where 'assist' ultimately comes from Latin 'adsistere' (via Old French 'assister'), and in Latin 'ad-' meant 'to' and 'sistere' meant 'to stand'.

'assistency' changed from Latin 'adsistere' to Old French 'assister', then into Middle English forms related to 'assistance', and finally into the modern English derivative 'assistency' formed by adding the suffix '-ency' to 'assist'.

Initially it meant 'to stand by' (literally 'to stand toward'), but over time it evolved into the current meaning of 'help or support'.