'predictably-decreased' originates from the combination of 'predictably' and 'decreased'. 'Predictably' comes from 'predict', which originates from Latin 'praedicere', where 'prae-' meant 'before' and 'dicere' meant 'to say'. 'Decreased' comes from 'decrease', which originates from Latin 'decrescere', where 'de-' meant 'down' and 'crescere' meant 'to grow'.

'Predictably' evolved from the Latin 'praedicere' through Old French 'predire', and 'decreased' evolved from Latin 'decrescere' through Old French 'decreistre', eventually becoming the modern English words 'predictably' and 'decreased'.

Initially, 'predictably' meant 'able to be predicted', and 'decreased' meant 'to grow less'. These meanings have largely remained the same in modern usage.