To carve deeply into recessed passages in order to clarify folds, openings, separations, and shadow.
Fouiller is a particularly useful French sculptor’s verb for working deeply into recessed parts of a statue—for example the folds of drapery, the interior of a hand, an open mouth, or other confined passages. A well-fouillé passage has sufficient depth and separation to make the form clear without destroying its structural integrity.
Etymology
French fouiller, 'to dig, excavate, search deeply.'
References
https://www.pierres-info.fr/glossaire_sculpture/index.html