For me It doesn't literally mean no rest, though I bet many who utters the cliche is themselves in the midst of a heavy bout with sleep depravity, all the while performing some meaningful work or task. Me being an intense admirer of sleep obviously refuses to brave such frontiers, but rather the cliche of 'no rest for the wicked' is a sort of motivating cry to keep moving it along. It's to combat laziness that I admit creeps into my inner thoughts with far too frequency.
No I should not watch that movie I've already seen a hundred times, nor should I skip exercising for today. We've got a finite amount of time in this world and it's paramount to always be moving forwards, improving constantly, and having different experiences. Revisiting what I've already been and done is a tremendous waste of time.
Rest is important, however. Sleep eight hours per day. At least.