Rapidly, Rapid
A sickness that hits you like an arrow will do its gross work rapidly. It's not a good thing, you're gonna get really sick really fast.
Picture this rapid sickness spreading through your body like a cartoon arrow ricocheting through your body. Yuck!
しつ
はや
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This rapid sickness makes your head pound like there's a shih tzu (しつ) yapping and barking nonstop. And it's making you sh*t a lot.
Imagine getting sick this rapidly. All of a sudden, there's a shih tzu barking in your head, it hurts! And... oh no, where's the toilet?!