It's a really old joke (shaggy dog story?) that I thought Foolestroupe might know. On reflection, as far in the past as Foolestroupe insists he is, maybe it's not an old enough joke for him to have heard it....
Of course kids like him sometimes worry about forgetting things they used to know, but he's probably past the time when he's even forgotten stuff he never did know, even though he is sort of a relative youngster 'mongst the truly well-seasoned of us here.