1. When someone says this they are describing a current situation that is not supposed to be easy.
2. A situation that cannot be solved easily.
3. Something that isn't a walk in the park.
4. Exclaiming that you are not satisfied with a situation.
Kyle: Dude lemme borrow your truck.
Marc: This aint no picnic!
Kyle: C'mon man it's Friday let's hit the weights hard today.
Marc: This ain't no picnic!