A TerribleProposal is a proposed rule change in a nomic that would be utterly destructive of the game. No player ever actually wants such a proposal to be adopted, so they are proposed only as a tactic with some ulterior motive, often a scam. Usually the usefulness of a terrible proposal revolves around the fact that players can be relied upon to work to avoid it passing. A merely bad proposal, by contrast, might be accepted or even promoted by players who dislike it, because they can accept the temporary inconvenience until it can be reversed.
NomicWorld saw a scam that involved a set of ten terrible proposals, which may be where the term was coined. At the time proposals could be seconded, and the seconder of a proposal was rewarded according to its electoral success. But interestingly, if one publicly refused to second a proposal, one would then be rewarded if it subsequently failed. The conspirators all publicly refused to second each other's terrible proposals, and when the proposals were inevitably voted down they picked up rewards for refusing to second that outweighed the penalties for proposing the failing proposals.
Occasionally a terrible proposal backfires by actually being adopted. This can be straightforwardly fatal to the nomic, though often it will prompt a search for a loophole or a judicial way to negate the effects. An example of a fatal proposal is Rishonomic, which replaced all numbers in the ruleset with the phrase "A Suffusion of Yellow". A non-fatal case is Agora's BlackRepeals, which purportedly repealed all the mutable rules, but which were eventually judged to not be valid proposals due to the absence of an oft-overlooked formality.