The word "feign" means "pretend." However, it also has an older meaning which is similar to "to fashion" or "to shape." In this case, the second one is correct, as Descartes is not claiming that something which he knows to be false is actually true. He is arguing that he does not know whether the world is real, nor whether he himself is real. However, out of the two, the possibility of him not being real is the hardest one to fathom. Therefore, although "pretend" is not a wrong approximation, the closest one is to "fathom" or "imagine."
This way of thinking led Descartes to argue that the one thing he knew for sure was that HE was real, because he was aware he had thoughts. This proposition is commonly known as "I think, therefore I am" (also, "Je pense, donc je suis" and "cogito, ergo sum").