* "Dumb" in this context doesn't refer to a lack of intelligence, but rather to the lack of a voice.
* "Waiter" refers to the traditional role of a person who waits on tables, serving food.
Therefore, a dumbwaiter is essentially a silent waiter, a device that carries food and dishes between floors without needing a person to physically carry it.
This term was first used in the 18th century, when the devices themselves were first developed. It highlights the comparison between the traditional human waiter and the new, silent, mechanical one.