A grouch is a cranky person who complains a lot. You could describe your grumpy old Grandpa as a grouch.
Use the noun grouch when you're talking about someone who's habitually in a terrible mood. If your usual bus driver is a grouch, he's probably always yelling at his passengers. You can also use grouch as a verb, to describe what a grouch does: grumble and gripe. The word grouch was originally 1890s United States college slang that might have come from grutch, "to murmur or complain."