JACK. Well, really, Gwendolen, I must say that I think that there are lots of other much nicer names. I thinkJack, for instance, a charming name. GWENDOLEN.Jack?...No, there is very little music in the nameJack, if any at all, indeed. It does not thrill. It produces absolutely no vibrations...I have known severalJacks, and they all, without exception, were more than usually plain. Besides,Jack is a notorious domesticity for John!
Got a boy back home in Michigan / And it tastes likeJack when I'm kissing him / So I told him that I never really liked his friends / Now he's gone and he's calling me a bitch again
Despite being a common noun, the word is still treated as a name and capitalized as such. The name is most often used with a descriptive "surname", showing the type of lad intended.