I recently heard somebody say that they "gave at the office" in response to a request from some charity. It also seems to have a more general usage when refusing a request for help of any kind. What exactly does this phrase mean, and what is its etymology? Is it a very recent saying?
The phrase originated from the United Way's workplace giving programs. Many people actually did donate to charity at the office and the point was that the person had already donated to charity as much as they felt that they should donate, and thus there was no reason to ask them to donate more money. However, it is now generally considered a sarcastic way of saying "Go away, I'm not going to donate any money."
This expression can be taken in two different ways, one of which has been explained and one that hasn't.
First, it can be literally true, as others have pointed out. When asked to give to a charity at home (either at the front door or via phone), someone who had already given to that charity via a workplace solicitation might answer, "I already gave at the office." (This can also be said dismissively or sarcastically when it's not actually truthful, but this disingenuous usage doesn't change the implication of the usage… this is merely what we call lying.)
Second, it can be used metaphorically, in the same general ballpark of the expression "busman's holiday." It can be used to beg off of a request to give one's time or energy to something – presumably as a volunteer – that is similar to what one does at work, especially when one's job is already in some level of service to others and with the rationale that one only has a limited amount of energy to devote to such tasks and that limited amount is already fully expended at work. An example might be a teacher being asked to tutor kids, or a nurse being asked to visit sick people. In this sense, "I gave at the office" means, "I already spend most of my days doing this task for others… I need to do something else on my time off."
I remember hearing a character say this in a cartoon from the bugs bunny related group of cartoons as a kid in the 70's as a way of saying "no, I've already done my part and given all I will so lay off and goodbye." It was clearly established and one of those adult inside jokes so common in those cartoons.