The meaning of crotchety is to be irritable or bad tempered.
For example if someone refers to you as a crotchety old man then they are basically saying that you're irritable or bad tempered and not being a nice person.
A crotchety old man would be an old man that would yell at kids to get off their sidewalk or stay away from their property even if the kids are not really doing any harm.
I had a neighbor who used to do that to me when I was a kid and I referred to him as a crotchety old man.
He would always yell at me for riding my bicycle down the sidewalk in front of his house or when I would walk down the sidewalk to get to my house.
I just ignored him and I was 7 and 8 years old then and my dad said the old man was a crotchety old man.