'rightfully-declared' originates from the combination of 'rightfully' and 'declared'. 'Rightfully' comes from 'right', meaning 'just' or 'fair', and the suffix '-fully', indicating 'in a manner of'. 'Declared' comes from the Latin 'declarare', meaning 'to make clear'.

'Rightfully' evolved from Old English 'riht', and 'declared' from Latin 'declarare', through Old French 'declarer'.

Initially, 'rightfully' meant 'in accordance with justice', and 'declared' meant 'to make known'. The combined term retains these meanings.