'customarily-united' originates from the combination of 'customarily' and 'united', where 'customarily' is derived from 'custom', meaning 'habit or tradition', and 'united' from 'unite', meaning 'to join together'.

'customarily' evolved from the Old French word 'costumier', and 'united' from the Latin word 'unitus', eventually forming the modern English term 'customarily-united'.

Initially, it meant 'joined together by habit or tradition', and this meaning has largely remained the same in modern usage.