A hard nut to crack. A difficult question to answer; a hard problem to solve. (Anglo-Saxon, hnut, a nut.) He who would eat the nut must first crack the shell. The gods give nothing to man without great labour, or “Nil sine magno vita labore dedit mortalibus.” “Qui nucleum essc vult, frangit nucem” (Plautus). In French, “Il faut casser le noyau pour en avoir l'amande.” It was Heraclies who said, “Expect nothing without toil.”
If you would reap, you also must plough
For bread must be earned by the sweat of the brow. E.C.B.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894