An inadequacy is something that is not enough. If you get a bad grade, it may reveal an inadequacy in your approach to studying.
The syllable qua in inadequacy rhymes with duh, and the accent is on the second syllable: "in-AD-uh-quh-see." You know that in- means "not," and the suffix y means "the quality or state of." But what is that middle part? Adaequare is the Latin word for "to make equal." So, inadequacy is the state of not being equal — or enough — for something.