Something that's underwhelming doesn't live up to your expectations. If your favorite soccer team doesn't play very well this year, you might say they're having an underwhelming season.
An underwhelming grade on your math test is a big disappointment, and an underwhelming movie by your favorite director isn't nearly as good as her last one. Anything underwhelming is a big letdown or major disappointment. Underwhelming and underwhelm are much more recent additions to the English language than the 14th-century overwhelm, on which they were modeled. Originally used in the 1800s, underwhelming was a humorous slang term well into the 20th century.