Unhelpful response to "Why?"

JUSTBECAUSE

["This casual phrase often shuts down further questions with its vague finality."]

The clue plays on the common phrase "just because," which is often used as a dismissive or unhelpful answer when someone asks "Why?" The answer is written as one word, "JUSTBECAUSE," to fit crossword formatting. It’s a straightforward example of how everyday expressions can become crossword answers.