Located, belonging, or designed to be placed under a pot.
Later, along with underpot mats, Grandma brought an old-line meat mincer, which she tried to install to the left of the uncomprehending Bruno. (Context: Among other things, Grandma Estela is bringing protective mats on which pots will be placed on the table prepared for dinner. Source: J∙ Okram - The Mystery of the Rammed Key.)
3rd person: underpots
To place in an insufficiently sized pot.