E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
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 labre dedit mortalibus. Qui nucleum esse vult, frangit nucem (Plautus). In French, Il faut casser le noyau pour en avoir lamande. It was Heracldes 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.