Presumptuous and presumptive come from Latin words meaning "to undertake beforehand," but they have different meanings in English. A presumptuous person is arrogant; he presumes too much; he takes too much for granted; he takes excessive liberties; he is bold: "It would be presumptuous of me to tell you how to handle your affairs." "To argue with an expert such as he is seems presumptuous." Presumptive, chiefly a legal term, means "based on inference," "not fully established," "affording only a basis for a supposition": "This case cannot be tried on merely presumptive evidence." "Until the will is read, your role as heir to the estate is presumptive."
Dictionary of problem words and expressions. Harry Shaw. 1975.