'explainableness' originates from Modern English, specifically formed from the verb 'explain' (ultimately from Latin 'explanare') plus the adjective-forming suffix '-able' and the noun-forming suffix '-ness', where Latin 'ex-' meant 'out' and 'planare' meant 'to make level/flat'.

'explain' comes from Latin 'explanare' which passed into Old French and Middle English (e.g. Middle English 'explainen'/'esplainen'), became the modern English verb 'explain', then took the suffixes '-able' and '-ness' in Modern English to form 'explainableness'.

Initially related to 'making level or plain' (literal sense), the root evolved to the abstract sense 'to make clear or intelligible'; today 'explainableness' denotes the quality of being capable of explanation.