Sarcastic “I bet!”

The crossword clue suggests that the answer is a sarcastic phrase that means “I doubt it.” The answer that fits this description is “as if,” which is often used in a sarcastic manner when someone says something that is hard to believe or untrue. For example, if someone says they won the lottery but didn’t bring any evidence, someone might respond with “as if!” The phrase “as if” is a contraction of “as though it were true,” and its use in a sarcastic way can imply disbelief or challenge the truthfulness of what was said.