It means grumpy, argumentative, "old bastard"
I find this quality very common in our industry. Both BOH, FOH and with diners too.
Is food service really just a place for all of us to find reasons to complain? Safely?
I always tell wait staff that there are people who go out to have bad time because it makes them feel better about their lives, so let them have a bad time, without compromising our standards, just do your normal and strive for perfection. Don't let them get to you.
Pessimists are actually happier than optimists. Optimists expect the best and are rarely experience that. Pessimist expect the worst and when that happens they can say "told you so" or if things turn out better than expected they are pleasantly surprised.