The difficulty or problem, as in We'd love to come but there's the rub—we can't get reservations. This expression may come from lawn bowling, where rub refers to an unevenness in the ground that impedes the ball. Its most famous use is in one of Hamlet's soliloquies (Hamlet, 3:1): “To sleep, perchance to dream—ay, there's the rub, For in that sleep of death what dreams may come ... Must give us pause.” [Late 1500s]