A terse retort. A short sharp response to someone who is struggling to contain themselves and is pushing their insecurities outwards onto everyone else. A 'grundy response' should only be used with people who you have a baseline friendship and / or respect with as they are often quite harsh and may include profanity.
The term can also be used in a derogatory fashion when referring to anytime you responded negatively to someone who is asking you to do something (haha I gave him a grundy response)
At a relaxed social gathering at Tommy's house people are preparing for a large scale group dinner. Tommy is stressed because he wants everything to be perfect, as a result he is pushing his stress outwards onto everyone else. Jim is making gravy for the above mentioned dinner:
Tommy: Jim is the gravy ready?
Jim: Not yet, I'm working on it, just need a little more time to make it good enough.
(30 seconds later)
Tommy: Jim, the gravy, is it ready? What's happening!?
Jim: I'm working on it, I want to do a good job, it should be ready soon.
(10 seconds later)
Tommy: Look, Jim, the consensus is that we don't care how good the gravy is.
Jim: Well, the consensus can go fuck itself Tommy <- Grundy response
At first look the Grundy response seems harsh, but in reality Jim is aware of how relaxed the situation is and that Tommy is just running around talking a lot but getting very little done other than stressing people out. The grundy response is used to snap him back into the reality of the situation.