Formal:
* Tough: This is a general term for something that's difficult to chew.
* Gristly: This describes meat that has a lot of cartilage, making it hard to chew.
* Fibrous: This describes something that's made up of tough, stringy fibers, like some vegetables.
* Indigestible: This word describes something that is difficult to digest, which often also means it's hard to chew.
Informal:
* Chewy: This can be used to describe something that requires effort to chew, but it can also have a positive connotation, like "chewy candy."
* Rubbery: This is used for something that's tough and elastic, like some meats.
* Leathery: This is used for something that's very tough and dry, like tough meat.
Descriptive:
* Hard as a rock: This is a very strong description for something that's extremely difficult to chew.
* Like chewing on a tire: This is a figurative expression that describes something very hard to chew.
Specific to Food:
* Undercooked: This can describe meat that's tough because it hasn't been cooked long enough.
* Overcooked: This can describe meat that's tough because it's been cooked too long.
The best word to use will depend on the specific context and what you are trying to convey.