Spinach is a type of food that appears in select tins. Eating it will provide 600 nutrition  (much more than other tins) and will increase your strength: "This makes you feel like Popeye!" (or "Swee'pea", if hallucinating)
The amount of strength increase is as follows:
|18 or less|| |
|18/85 or more|| |
Cursed tins of spinach contain a “decaying green substance” which makes 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.