Wikipedia has a thorough explanation of 'sunk cost', which mentions the 'sunk cost fallacy':

Many people have strong misgivings about "wasting" resources. This is called "loss aversion". Many people, for example, would feel obligated to go to the movie despite not really wanting to, because doing otherwise would be wasting the ticket price; they feel they passed the point of no return. This is sometimes called the sunk cost fallacy. Economists would label this behavior "irrational": It is inefficient because it misallocates resources by depending on information that is irrelevant to the decision being made.

This line of thinking, in turn, may reflect a nonstandard measure of utility, which is ultimately subjective and unique to the consumer. When a ticket-buyer purchases a ticket in advance to a bad movie, he has still made a semi-public commitment to watching it. The ticket-buyer may "save face" by sticking it out, a satisfaction he cannot draw if he leaves. To leave early is to make his lapse of judgment manifest to strangers, an appearance he may rationally choose to avoid. He may, in fact, find some amusement in how bad the movie turned out to be, and take pride that he recognised it to be bad. Or he may feel qualified to criticize the movie in front of his peers. Either way, this mitigates the decision to view the movie, not the decision to purchase the ticket.

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So in poker, I suppose it's the idea that when you've bet a large portion of money, you somehow 'should' continue to bet. The smart, cool player, who does not buy in to the sunk cost fallacy, will fold when he or she needs to, regardless of what came before.

Then again, I don't play poker myself...