An irrational idea is an idea that is not currently anchored in empirical evidence or rational proof. It might be speculative. It might be fictional. It might be symbolic, spiritual, creative, confused, or simply untested. The key is not mockery. The key is classification.
In TST, irrational does not automatically mean stupid. It means the idea has not yet earned empirical or rational status. A unicorn is irrational if presented as a real animal without evidence. Valhalla is irrational if presented as an actual realm without evidence. A new scientific hunch may also begin as irrational before it is clarified, tested, and possibly moved into the rational or empirical category.
This is important because many good ideas begin as wild ideas. They start as hunches, guesses, dreams, questions, or strange possibilities. Most fail. Some become useful. A few become part of common knowledge.
The discipline is to hold irrational ideas properly. Enjoy them. Explore them. Use them in fiction, metaphor, art, spirituality, or speculation. But do not confuse them with tested truth until they survive the right kind of testing.