Someone is clueless if they have no idea what's happening. The unsuspecting victim of a practical joke is usually clueless right up until everyone starts laughing.
You could reassure your brother that your mom has no idea you're planning a surprise party by saying, "Don't worry, she's totally clueless." Or you might worry that your friend failed the big geometry test, based on the clueless expression on his face during the exam. In the 1860s, clueless meant "leaving no clues behind," but by 1943 it gained its current meaning, which became even more popular as student slang in the 1980s.