Put this clip at the very start of a reaction sequence, before any dialogue, then follow it with the bruh sound effect for a deadpan reply or the baby laughing meme sound effect when the room cracks up instead.
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A small child says hello motherfucker in a clear, high-pitched voice in just over a second, with no music behind it. The effect is funny and startling because the delivery sounds sweet and confident while the words are bluntly rude. It became known as a Vine meme and spread from there into short-form edits.