In
NetHack, a tin contains something
edible, usually
meat but occasionally
spinach. They can be found lying around the
Dungeons of Doom, or you can create them yourself with a
tinning kit. A tin will take several turns to open, so if you are
starving to death in the middle of a
melee (and it happens much more often than you think,
non-Hackers), a tin is probably not the best choice of
comestible. If you
wield the otherwise-useless item called a
tin opener, you can open the tin in a single turn. And if the
tin opener was
cursed, you're in for
big problems.
Tins can contain rotten meat, but this is not the same as tainted meat -- meat eaten from a tin will not make you sick (although I once modified NetHack to make my ascension-obsessed roommate's characters start off with tins of tainted Wizard of Yendor meat. It was pretty kludgy but I got him good the first time).
A tin of spinach will increase your strength, naturally. Remember, the DevTeam thinks of everything.