If you are grouchy, you are in a bad temper, especially because you are tired — short with people, ready to complain, not much fun to sit next to. It is a homely, informal adjective, originally American college slang from the 1890s, and it still sounds casual and a bit fond, the way a family talks. You can be grouchy at any age; though actually, English often hangs it on mornings, hunger, heat, and old people in stories, so students sometimes think it only belongs to grumpy grandfathers. It is milder than furious and closer to grumpy. It describes a passing mood more than a fixed character, unless you call someone a grouch, which is the person who is like that a lot.