When you hear the word presumptive, you might automatically think it has something to do with being presumptuous. However, the two words have different meanings.
Presumptuous describes someone who is excessively confident or bold, especially when they are not entitled to be.
Presumptive refers to something that is based on assumption or conjecture. It often describes a person who is acting on assumptions, or it might describe an idea or event that is presumed to happen based on probability or reasonable belief.
These two adjectives are both derived from the verb presume, which means “accept as true or appropriate without sufficient evidence,” including ideas, situations, and one’s own actions:
I shall presume that your data is valid, for the moment.
Ralph presumed to tell people what to do, with poor results.
In the first sentence, presume means “accept as true”; you could say that I consider the data to be presumptive facts, or presumptively true; I accept the truth of the data without evidence.
In the second sentence, presume means “(dare to) act in a way which is not known to be appropriate.”
Ralph has no authority, but he orders people anyway; he is presumptuous.