To subject clay or glaze to insufficient heat work in the kiln. This prevents the clay body from fully vitrifying or the glaze from melting completely. It can be an accidental result of a stalled kiln or poor heat distribution, but potters sometimes do it intentionally to achieve a porous, matte, or dry surface texture.
Heard on the job
If you underfire that stoneware, it will weep water on the table.