completely contrary to nature, reason, or common sense; utterly foolish; absurd; senseless: a preposterous tale.

What is a preposterous person?

: very foolish or silly. See the full definition for preposterous in the English Language Learners Dictionary. preposterous. adjective. pre·​pos·​ter·​ous | \ pri-ˈpä-stə-rəs \

Is preposterous an insult?

If you describe something as preposterous, you mean that it is extremely unreasonable and foolish.