These two might seem like straightforward antonyms, but there are a few differences to keep in mind. Overwhelm is a verb that means “to overpower” or “to cover or bury.” Underwhelm means “to fail to impress.” Basically, these words have opposite meanings.
Overwhelm is a versatile verb. A situation can overwhelm someone. That same person can be overwhelmed by a situation. They might describe the situation as overwhelming. This is the same scenario described three ways, but in each case, the situation can be said to overpower the person in question.
Another definition of overwhelm is “to overcome completely in mind or feeling.” This tends to refer to a person’s emotions. For example, a parent at their child’s college graduation might be overwhelmed by pride.
Another definition of overwhelm is “to overpower or overcome.” This definition can also mean to destroy or crush. For example, an unprepared political candidate might see their campaign overwhelmed by a stronger opponent.
Overwhelm can also have a more physical definition: “to cover or bury something.” This can often refer to natural forces, like a flood or an avalanche. Lava might overwhelm a town sitting at the foot of a volcano.
Lastly, overwhelm also means “to overload a person or thing with large amounts of anything.” This one can be applied to many different situations. For example, a child might be overwhelmed by presents on her birthday (meaning she has more than she can handle). A celebrity might be overwhelmed by questions from reporters (again, meaning there are more than they can handle). A company that recalls a defective product might be overwhelmed by complaints and phone calls from customers.
The verb underwhelm has a much more limited use. It means “to fail to interest” or “to make no positive impression.” While overwhelm has been used for centuries, underwhelm is a much newer word. It’s only been around since the 1900s.
Overwhelm can refer to many different physical and mental situations. Underwhelm, however, usually only refers to people’s feelings toward something. Critics might find a widely hyped movie to be underwhelming (or disappointing). An unhappy couple might feel underwhelmed by their relationship. A young person who moves from a big city to a small town might find the new town underwhelming.
Both overwhelm and underwhelm come from the older word whelm. Whelm can mean to submerge, or it can also be a synonym for overwhelm. Someone can feel whelmed with happy feelings, for example. And, yes, whelm is still (rarely) used today.