This page deals with slang terms for a Zombie apocalypse, in the heat of the moment it's easy to garble in horror at the situation and important messages are lost so easy terms are introduced by the military which are inevitably adopted and mutated by civilians.
In art, an underpainting is an initial layer of paint applied to a ground, which serves as a base for subsequent layers of paint. Underpaintings are often monochromatic and help to define colour values for later painting.
Undermining: when the depth of the wound extends past the original opening. Sort of like a plastic water bottle. It differs from tunneling in that tunneling is usually in one direction, whereas undermining extends past the opening in all directions.