A crumpdown is when you have the last 4 crumpets, they're absolutely fucking lush and then you finish them. They're so nice that you want more, but there aren't any crumpets left so you're left helpless wanting for more.
Symptoms are: Shaking, cold sweats, increased heart rate, slurring of words and blurred vision
"Duude, I need a fix I'm crumping down pretty hard, make it extra buttery. This is the worst crumpdown ever!"