The term 'grouch' implies a negative judgment about someone's personality or mood. I didn't talk to him because I knew he was being a grouch today.
Pro Tip 2/3
Not Always Serious
'Grouch' can be used lightly among friends when someone is temporarily ill-tempered. You missed breakfast, huh? No wonder you're such a grouch this morning!
Pro Tip 3/3
Descriptive Imagery
Imagine someone with a scowl or frown; that visual might indicate a 'grouch'. Why is that grouch always frowning whenever we have a class meeting?