This Latin phrase is usually translated into Anglo-Norman as (de)suz le pé de seal, see sub pé1.
s.
sub pede sigilli
1lawcertified by the half-seal (impression of the reverse side of the Great Seal):
(
1324-25
)
et les roulles le dit Sire Henri des pleyntes ore de novel faitz, soient maundez sub pede sigilli devaunt ceus qe serront ore justices assignetz
i 416