'customarily-divided' originates from the combination of 'customarily' and 'divided', where 'customarily' is derived from 'custom', meaning 'a traditional and widely accepted way of behaving or doing something', and 'divided' from 'divide', meaning 'to separate into parts'.

'customarily' evolved from the Old French 'costumier', and 'divided' from the Latin 'dividere', eventually forming the modern English term 'customarily-divided'.

Initially, 'customarily-divided' meant 'divided according to custom', and this meaning has largely remained the same in modern usage.