The adjective fantastic has two meanings — extraordinarily brilliant or ludicrously far-fetched. So when your boss calls your suggestion of work-at-home-in-your-bathrobe-Fridays for the whole office fantastic, be sure you know which one he means.
We get fantastic from the Latin phantasticus, meaning “imaginary.” Sometimes it’s still used that way: If you call a unicorn a fantastic beast, you’re not paying it a compliment; you’re saying it exists only in fairy tales. Most often, though, fantastic means strikingly out-of-the-ordinary. It can be complimentary ("You got an A? Fantastic!") or disparaging ("He was always foolishly unrealistic, but trying to reach the moon using a balloon tied to his bicycle was his most fantastic idea yet.").