1. Concerning plants and animals that have or are covered with barbs or quills: Good examples of spiny animals and plants can be the sea urchin, fish with spiny fins, hedgehogs, a spiny cactus, or even the rosebush stems which have lots of thorns, all of which are to protect them from predators!
2. Pertaining to something which is difficult to handle or to understand: For her math homework Mary had a very spiny or puzzling problem to solve for the next day.
3. Regarding something which is shaped like a thorn or barb: The crochet hook little Timmy found in his mother's sewing box was quite thin and spiny and looked dangerous!

The grasshopper sitting in the sun had two thin spiny legs, ready for jumping.