When your friend suggests a restaurant but you’re in a bad mood so you say “That’s a stupid restaurant” — even though you don’t really mind it — then you’re being grumpy, meaning irritable or grouchy.
There are all kinds of reasons for feeling grumpy: maybe you’re tired or annoyed or you have a headache. Whatever the cause, when you’re grumpy you just want to sulk in a corner. Even though it’s a negative word, it’s a pretty gentle one. When you're grumpy, you’re not mad or mean — you’re just unpleasant for the moment. But don’t tell someone who’s grumpy that he's being grumpy . . . He likely won’t respond kindly to that.