A lot of things are used in a different way than that their function is. Look at the poor screwdriver, he spends more time opening paintcans than actually screwing ;)
I looked it up and whet in the proverb here, is used in the same way as with whetsstone, meaning to sharpen. (pretty sure wit is related, which is related to weten/wissen (to know) in dutch/german. To stimulate or sharpen ones mind or appetite.