Cranky is one of those words that sounds a little bit like what it means: one who is cranky is easily annoyed, irritable, or testy. My sister gets cranky when she doesn't eat. If you want to have a good day, keep her well fed.
A cranky person doesn't get furious or outraged, they're just easier than usual to annoy—things get on their nerves more quickly than they do for other people. You could think of someone who's cranky as having actual crank. When they're hungry, tired or otherwise worn out that crank is easy to turn—it doesn't take much to make them mad. A person in a good mood, though, has a crank that isn't as easy to turn. That's because they're not cranky.