From Anglo-Norman -if (feminine -ive), from Latin -ivus. Until the fourteenth century all Middle English loanwords from Anglo-Norman ended in -if (compare actif, natif, sensitif, pensif etc.), and under the influence of literary Neolatin both languages introduced the form -ive. Those forms that have not been replaced were subsequently changed to end in -y (compare hasty, from hastif, jolly, from jolif etc.).
Like the Latin suffix -io (genitive -ionis), Latin suffix -ivus is appended to the perfect passive participle to form an adjective of action.