Bouvier's Law Dictionary, Revised 6th Ed (1856):

FRANK-ALMOIGN, old English law. This is a French law word, signifying free-
alms.
     2. Formerly religious corporations, aggregate or sole, held lands of
the donor, to them and their successors forever, in frank almoign. The
service which they, were bound to render for these lands was not certainly
defined; they were, in general, to pray for the souls of the donor; his
ancestors, and successors. 2 Bl. Com. 101.