Pirate, I didn't know the answer myself so I went to wikipedia to check it out. Thanks for your interesting comment, by the way.
I'd heard about it every Halloween and stuff but never really cared to pay attention of where the word was originally from. Anyway, if you wanna know more about it, this is the link you can check it out,
This is a part of it for anyone who can't see the link,
"The practice of carving jack-o'-lantern goes back to the Irish legend of Jack, a lazy but shrewd farmer who tricked the Devil into a tree, then refused to let the Devil down unless the Devil agreed to never let Jack into Hell. The Devil agreed. When Jack died, he was too sinful to be allowed into Heaven, but the Devil wouldn't let him into Hell. So, Jack carved out one of his turnips, put a candle inside it, and began endlessly wandering the Earth for a resting place. He was known as Jack of the Lantern, or Jack-O'-Lantern." (copied from wikipedia.com)
And this is the link about Halloween if anyone is interested in knowing more about it,