Marrow melted out of bones and beaten to a cream as it sets.
Hartley 1954 suggests this as "excellent for thin nervous children", but it is now more common as a baste for beef steaks.
See also:
The similar:
Clarified Marrow
and...
Marrow Toast
Marrow Toast 'Victoria'
Vegetable Marrow Butter
For other marrow dishes, see: Bone Marrow