The hummingbird was named for the sound that bird makes in flight which sounds like a hum. A hum is a sound made by producing a wordless tone with the mouth closed, forcing the sound to emerge from the nose. To hum is to produce such a sound, often with a melody.
See also: The Hum, Hummer, Humming (disambiguation), The Hummer, Hums, and Hummingbird.