The word humdrum is an example of grammatical reduplication or repetitive rhyming, much like other informal words such as itsy-bitsy, okey-dokey, hoity-toity and namby-pamby. Humdrum actually means a boring, dull or monotonous state of being, with little hope of spontaneity or excitement. Living in a small rural city with very few sources of outside entertainment could easily be described as a humdrum existence. A routine factory job or a monotonous clerical position would also be considered humdrum. The expression is very similar to the informal word ho-hum, which also describes a boring or dull set of circumstances.
The "hum" in humdrum or ho-hum has long been used to suggest the exasperated humming sound inspired by boredom. Many people punctuate their feelings of boredom by vocalizing a long, drawn-out sigh or yawn. As with other reduplicative words, the "hum" is paired with a rhyming non sequitur, "drum." In essence, living a humdrum lifestyle would naturally lead to the stifled humming sounds of a yawn. Some sources suggest the use of humdrum to describe such a dull and ordinary situation can be traced back to at least the 16th century AD. Many popular reduplicative or rhyming slang words can also be traced to the informal speech patterns of commoners during that time period.
Occasionally the word can be found separated into two parts, hum drum, but this tends to mislead the reader into thinking there may be an actual drum associated with the etymology. An argument could be made that the rhyming word "drum" was not selected at random, however. Some people do drum their fingers or perform drumming riffs during times of extreme boredom, but there is precious little evidence to back up such speculation. There is also the idea of "drumming" up some excitement when confronted with hours of tedium or boredom. Again, there is no official evidence to support any other claim except for a satisfying rhyme with "hum." An actual drum used to combat the effects of tedium has yet to be discovered.
There are other phrases which also cover the same general mediocrity landscape as humdrum. A person could become "bored to tears" while performing a mundane or repetitious task, for example. A rural area with few social or recreational outlets could be described as a "one horse town." A monotonous job is often described as being "as exciting as watching paint dry." An less-than-interesting movie or book could be described as "ho-hum" or "dull as dishwater." A long, dull day may move "as slow as molasses." There are many ways to describe an exceedingly dull or uninspired existence, and humdrum is just one of them.