A trust parasite is any entity (an individual or organization) that is only able to sustain itself by being parasitical on (i.e. taking advantage of) the social resource of trust. In the process of doing this, it erodes trust overall, "ruining things for everyone" by requiring everyone to be more vigilant against other trust parasites.
This is similar to a "con game", except that a trust parasite does not go to the trouble of building up confidence first; it merely capitalizes on an existing default.
In order for any society to function, there must exist a certain base level of trustworthiness that is assumed, absent evidence to the contrary. (This is a consequence of the iterated prisoner's dilemma.)
A trust parasite capitalizes on this assumption by making agreements (formal or informal) to provide something in return for something given to it, but then delivers less than expected. This allows it to spend less of its own resources than a more honest entity would need to do, while still receiving the same level of compensation.