Powell's Principle of Non-Gluteality
An argument sometimes applied to Lojban semantics by Robin Lee Powell, which goes something like this:
X (a Lojban construct) could in principle mean any of Y or Z. But if you use X to mean Y, you'd be an ass. The listener should assume that you are not being an ass, and that you really mean Z.
Arguably, this is a rediscovery of Grice's cooperative principle.