the.com/filling
The stuff in the middle that makes the outside worth biting into.
means A substance packed inside something else — food, teeth, cushions, or pillows — to occupy space or hold structure.
from From Old English fyllan, to make full, rooted in the same source as full; the gerund became a noun for whatever does the filling.
Dental debutFirst metal tooth fillings date to ancient Italy.
Double meaningIt both satisfies hunger and patches cavities.
Volume wordOriginally meant the act, not the contents.