A household word is any word (or phrase) that is widely known in a culture. Obviously, the phrase implies that the word is so common, you would not be surprised to hear it in your household.
But when did household word become a household word?
You can thank William Shakespeare. Apparently, the guy invented a lot of phrases, and this was one of them. In Henry V, Act IV, Scene 3, during the famous "band of brothers" speech, Henry V says
Old men forget: yet all shall be forgot,
But he'll remember with advantages
What feats he did that day: then shall our names.
Familiar in his mouth as household words
It's a neat turn of phrase, because it goes hand in hand with the Q index (the rating of pop culture). It means that someone or something has become such a big deal that people can't help but talk about it over the dinner table. I remember when you couldn't go three hours without hearing the phrase "Star Wars." That franchise was some hot stuff. Jedi became a household word.
Some people use the phrase disparagingly, implying that the new thing isn't worthy of discussion. "Somehow or other, N Sync became a household word." Whatever the case, household words reflect on society as a whole. After all, culture begins the home.