toughie

A toughie is something difficult to deal with, solve, or answer—a hard problem, a challenging question, or a demanding situation.

That last question on the exam was a real toughie.

The word can also refer to a person who is rough, aggressive, or physically intimidating, though this sense is less common now.

Toughie carries an informal, slightly playful tone, often used when acknowledging difficulty without taking it too seriously.