Vain Imaginings are those useless thoughts that go through a persons mind when they are being useless. The term is commonly used in religious documents. That's because God doesn't want people sitting on their ass doing nothing all day.
In political debates, when someone asserts an opinion that cannot be supported by evidence--usually something that has been debunked by fact-checkers for several years and is often in Alex Jones territory--we should respond that we're not obligated to "debate your imagination".
You claimed President Obama is a Muslim born in Kenya as a Manchurian Candidate in order to ruin the Constitution, confiscate all guns, lock up Christians in FEMA camps, and usher in the reign of the Anti-Christ, but you haven't shown evidence that it is a reality that exists outside your own head. Why do you think we're required to debate your imagination?
When you say that someone has a wild imagination means that this person has the ability to think and dream about anything; this person has very vivid and colorful thoughts