'externally-assessed' is a compound formed from 'externally' and 'assessed'. 'externally' derives from Latin 'externus' (via Middle English/Old French), where 'externus' meant 'outside' or 'outer'. 'assessed' derives from English 'assess', ultimately from Latin (see below).

'externally' entered English via Old French and Middle English from Latin 'externus'. 'assess' entered English via Old French/Medieval Latin (e.g. Old French assesser / Medieval Latin assessare) from Latin roots related to 'assidere' ('to sit by'). The compound form combining an adverb with a past participle (e.g. 'externally assessed') became common in modern administrative and academic English to describe evaluations conducted by outsiders.

Individually, 'external' originally meant 'outside' and 'assess' historically related to sitting beside (judges or tax officials) and determining value; together the modern compound came to mean 'evaluated by someone outside the organization', a use that developed with institutional quality assurance and external examination practices.