Spinach is a type of food that appears in select tins. Eating spinach will provide 600 nutrition if blessed or 401-600 nutrition if uncursed  (much more than other tins) and will increase your strength: "This makes you feel like Popeye!" (or "Swee'pea", if hallucinating). Eating cursed tin of spinach will provide 201-600 nutrition but decrease your strength.
|17 or less|| |
|18/85 or more|| |
Cursed tins of spinach contain a “decaying green substance” (or “decaying substance” if you are blind). If you decide to eat it, you lose the amount of strength you would otherwise have gained.
The use of Spinach and its effects in NetHack is a reference to comic strip character Popeye, who gains incredible strength after ingesting the vegetable.