Where does underrated come from?

The first records of the verb underrate come from the 1600s, but for much of its history it was used in a technical sense related to the valuation of property or the estimation of taxes.

It can still be used this way, but as an adjective it is popularly used to express the opinion that the general consensus (the popularly held opinion) about something isn’t as favorable as you think it should be. Just about anything can be described as underrated, but it is often applied to aspects of popular culture. It’s frequently used to describe not things that are generally disliked but things that a lot of people like but don’t love. You might describe an actor as underrated if you think their performances are better than people give them credit for. You might argue that a movie is underrated when you think it’s awesome but other people think it’s just OK. Pro athletes are especially prone to being labeled as overrated or underrated. Sometimes, underrated can be used to refer to literal ratings, as in Critics severely underrated her last album, rating it an average of three stars when it’s clearly worthy of five. Occasionally, underrated is used to describe a thing that is universally loved to emphasize how great it is (and to be funny or ironic), as in I gotta say, pizza is underrated.

Saying something is underappreciated is similar to saying it’s underrated, but underappreciated can imply that not enough people know about a thing to appreciate it (or fully understand its importance), whereas underrated usually only relates to the existing popular opinion about a thing.