hum (v.) Look up hum at Dictionary.com
late 14c., hommen "make a murmuring sound to cover embarrassment," later hummen "to buzz, drone" (early 15c.), probably of imitative origin. Sense of "sing with closed lips" is first attested late 15c.; that of "be busy and active" is 1884, perhaps on analogy of a beehive. Related: Hummed; humming. Humming-bird (1630s) so called from sound made by the rapid vibration of its wings.
There is a curious bird to see to, called a humming bird, no bigger then a great Beetle. [Thomas Morton, "New English Canaan," 1637]
hum (n.) Look up hum at Dictionary.com
mid-15c., from hum (v.).