'twenty-fifth' comes from the combination of 'twenty' and 'fifth'. 'twenty' originates from Old English 'twentig' (meaning 'two tens'), and 'fifth' originates from Old English 'fifta'/'fifþa' (meaning 'fifth').

'twentig' and 'fifta' evolved through Middle English forms (e.g. 'twentie', 'fifthe') into the modern English words 'twenty' and 'fifth', which later combined as the compounded ordinal 'twenty-fifth'.

Originally the elements meant 'two tens' and 'the fifth'; combined they have consistently expressed the ordinal idea of 'the fifth after twenty', which developed into the simple modern meaning '25th in order'.