Undermining is a word you may hear from a wound care team, or see written on a wound assessment form. This article can’t tell you exactly what your own team means by it. What it can describe is a wound with space underneath the skin.
Packing a wound with a wet-to-dry dressing treats that space as routine: place the gauze pads or packing tape in your wound, and carefully fill in the wound and any spaces under the skin. So a space beneath the skin at the edge of a wound is something standard home dressing instructions expect, even where the clinical term is never used.
Here’s why the space is filled rather than left. An open wound will heal from the inside to the outside. A dressing helps absorb drainage and keeps the skin from closing before the wound underneath fills in. The point of packing a space is to keep the surface from sealing over something that has not yet filled in.
The most useful thing you can do with the word is to ask the person who used it. Ask your wound care team to show you, on your own wound, what they are describing, where it is, and whether it changes how your dressing should be done or how often.
This answer deliberately does not give you a measurement, a grade, or a rule for when a space under the skin becomes serious. There isn’t one to give here, and a made-up number would look like guidance when it is not.