Riddikulus comes from the Harry Potter series, where it is the charm used to force a boggart to take a funny, harmless shape instead of the thing a person fears most. That idea of turning fear into laughter made the word easy to reuse outside the books and films, especially in short clips and edits where a tense setup needs a comic release. On soundboards it survives as a compact reference that Harry Potter fans recognise at once, while other listeners still get the joke from the timing and the change in tone.