'pediatric-onset' originates from modern English, specifically from the combination of 'pediatric' (from Greek roots) and 'onset' (from Old/Middle English). 'pediatric' ultimately comes from Greek 'paidiā'/'paidos' meaning 'child' and 'iatros' meaning 'physician' (via Latin/French medical terms), while 'onset' comes from Old English/Middle English elements 'on-' + 'set' meaning 'a setting on/beginning'.

'pediatric' developed from Greek components ('paidos' + 'iatros') into Latin and Romance-language medical terms (e.g. Latin/Medieval forms and French 'pédiatre'), then entered modern English as 'pediatric' (US) / 'paediatric' (UK). 'onset' evolved from Middle English forms such as 'onsetten'/'onsetten' and the combination 'on' + 'set' into the modern noun 'onset'. These parts were later combined in modern English usage to form the compound 'pediatric-onset'.

Individually, the roots referred to 'child' and 'physician' and to the idea of 'setting on' or beginning; over time the combined modern compound came to mean specifically 'beginning during childhood' when describing diseases or conditions.