A more thorough explanation:

Wainable means that land can be plowed or tilled. It is an old-fashioned word that is not used much anymore.

• The farmer had a lot of wainable land, so he was able to grow a variety of crops.
• Because the soil was so rocky, the land was not wainable and could not be used for farming.

The first example shows how wainable land can be used for farming. The second example shows how land that is not wainable cannot be used for farming because the soil is not suitable for plowing or tilling.