In most cases, the meaning of a word depends on its root word or basic unit of a word and the prefixes and suffixes added to the root word as all of these combine and create a new word and meaning. In the case of the word "improvise" this word contains two prefixes that are "im-" that means "not" and "pro" that means "pre-" as stated in the excerpt from the question and the root word "vis" that means "to see" or "to observe" therefore by combining all of these parts the word "improvise" means "not foresing" which is commonly used to describe actions or similar that were not foreseen or prepared. This means it is the root word "vis" the one that provides the missing part and meaning and therefore the word that means "not foreseeing" is "improvise".
