sharp as a tack (but in the sense that you're not smart, just a prick)
S4 is fairly confident that when the doctor tells him to pack his things, tells him he's being relocated to more appropriate quarters, tells him it's nothing to do with him, obviously, because the provided conditions have been more than adequate, but that it's instead due to some impending renovations on the otherwise-vacant Guard barracks...
Well, he thinks, really, that Gaster is very nearly convinced he's doing his subject a favor.