'onset' originates from Middle English, specifically the compound formed from 'on' + 'set', where 'on-' meant 'on/against' and 'set' (from Old English 'settan') meant 'to place or put'.

'onset' developed from the Old English verbal elements (on- + settan 'to set') through Middle English compounds such as 'onsetten' or 'on-seten', eventually becoming the modern English noun 'onset'.

Initially it referred to the action of setting on or an attack ('setting on'); over time its meaning broadened to include the general idea of a beginning or start as well as an attack.