Kayyūm is the intensive form of the word kāʾim. Its original morphological form is kayvūm in the pattern of fey'ul. Because the first of the consecutive letters wāw and yāʾ is unvocalized, the wāw has been changed to a yāʾ. As for the meaning of the word kayyūm: the author of al-Kashshāf (al-Tahānawī) says the following on this matter: Kayyūm means one who is self-subsisting and everlasting. Here, its use in the intensive form conveys two senses: abundance in quantity (kammiyyat) and abundance in quality (kayfiyyat). Kayyūm means the one who preserves, watches over, and perfects everything. Indeed, the following statement of Allah Most High conveys this meaning: "Is He who watches over every soul [nafs] as to what it has earned [like any other]?" (al-Raʿd 13:33).
I say: According to the apparent meaning of the phrase, qiyām (standing, persistence) signifies continuity. By attributing a transitive sense to the word, it is used to mean causing to continue. This, too, means to preserve. (Hayākil, 94)