'nationally-assessed' is a compound of 'nationally' and 'assessed'. 'Nationally' is formed from 'national' + the adverbial suffix '-ly'; 'national' ultimately comes from Latin 'natio' meaning 'birth, nation' via Old French 'national'. 'Assessed' is the past participle of 'assess', which comes from Latin 'assessare' (a frequentative of 'assidere') via Old French 'assesser' and Middle English.

'Nationally' was formed in modern English by adding '-ly' to 'national'. 'Assess' entered English through Old French and Medieval Latin (from Latin roots 'assessare'/'assidere') and became the verb meaning 'to evaluate'; the past participle 'assessed' has been used adjectivally (e.g., 'assessed value') since Middle/Modern English. The combined phrase 'nationally assessed' appears in administrative and educational contexts from the 19th–20th centuries onward.

The root 'natio' originally referred to 'birth' or a group of people born in the same place; 'assidere' originally meant 'to sit beside' (in contexts of judging or estimating). Over time 'national' shifted to mean 'of or relating to a nation' and 'assess' shifted to mean 'to evaluate or estimate', yielding the modern sense 'evaluated at a national level'.