'childhood-onset' is a modern English compound formed from 'childhood' and 'onset'. 'childhood' ultimately comes from Old English 'cildhād', where 'cild' meant 'child' and '-hād' meant 'state' or 'condition'; 'onset' comes from Middle English 'onsetten', where the prefix 'on-' meant 'on' and 'setten' (from 'set') meant 'to place or set (often to set upon/attack)'.

'childhood' changed from Old English 'cildhād' to Middle English 'childhode/childhude' and eventually to modern 'childhood'; 'onset' evolved from Middle English 'onsetten' (action of setting on/attack) into the noun 'onset' meaning 'beginning', and these elements combined in modern English to form the compound 'childhood-onset'.

Initially, 'childhood' meant 'the state of being a child' and 'onset' originally implied 'an onset or setting on (attack)'; over time 'onset' shifted to mean 'beginning', so the compound now means 'beginning during childhood'.