Difficult, unrewarding, tough or nearly impossible to accomplish. Orson Welles did a commercial spot for frozen peas, during which he was required to say "crumb-crisp coating", at which he remarked, "ooh, that's tough."
Lois: How did the hedge-trimming go?
Nick: It was crumb-crisp, I'll tell you. I had the wrong sized blade and there were many biting flies. But I finished at last.