Tax
A guard is at your door and wants some of your grains. This is the tax you pay for living in this town. Back in the day, grains were used like currency, so it makes sense that there was a grain tax in the olden days.
Can you imagine a guard coming to your door and demanding you pay your grain tax?
ぜい
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You have to pay this tax, because if you don't the guard will cut you down with his special German saber, a zeiber (ぜい). "Don't you hear vat I zei? Pay ze tax or I vill zlei you!" he shouts.
You don't want to be sliced with the zeiber, so you listen to vat he zei zei zei's and pay the tax.