Unsafe Enchant signs allow you to enchant the item that the user currently holds. It does not have any level restrictions on enchantment levels. That means that even sharp:500 would be a valid enchantment.

1. [UEnchant]
2. enchantment:level
3. (Continuation)
4. Optional: The price

You can find a list of enchantments and abbreviations on the Enchantments page.

[UEnchantment]
baneof:50

This would enchant the item with Bane of Arthropods 50.