Could you please tell me the precise meaning of "fantastic" in this excerpt of the Chapter 7 of The Great Gatsby? I'm French, so, if you could also give a synonym in English of this word in this context, it would be nice of you:
I glanced at Daisy, who was staring terrified between Gatsby and her husband, and at Jordan, who had begun to balance an invisible but absorbing object on the tip of her chin. Then I turned back to Gatsby—and was startled at his expression. He looked—and this is said in all contempt for the babbled slander of his garden—as if he had “killed a man.” For a moment the set of his face could be described in just that fantastic way.
In English, there are a variety of definitions of "fantastic." It can mean unreal (as in fantasy), strange, remarkable, incredible, wonderful, unrestrained, exotic, grotesque, fanciful, and illusory (an illusion). With so many different definitions, each particular use of the word definitely depends on the context.
At this point of the novel, everything is out in the open about Gatsby's and Daisy's affair. Tom and Gatsby are firing back at each other. Tom has accused Gatsby of having achieved his money by working with crooks like Wolfsheim (which is, of course, true). Then Tom accuses Gatsby of recently cooking up an even larger, illegal scheme. Everyone in the room can feel the tension. Nick looks at Gatsby and can't believe the look on his (Gatsby's) face. Gatsby is angry and desperate to defend himself, especially with Daisy looking on. He looks as if "he killed a man."
In the context of the rest of the quote from The Great Gatsby, "fantastic" could mean grotesque (angry and monstrous) but it more likely means "unbelievable." Despite Gatsby's flaws, Nick has a high opinion of Gatsby's genial nature and romantic idealism. Nick thinks it would be unbelievable for Gatsby to have a look as if he had just "killed a man." But Gatsby is that upset, so his look of anger and desperation (as if he had killed someone) is unbelievable. So, in this context, a synonym of "fantastic" could be "unbelievable":
For a moment the set of his face could be described in just that unbelievable way.