The Collaborative International Dictionary of English v.0.48:

Alfet \Al"fet\, n. [LL. alfetum, fr. AS. [=a]lf[ae]t a pot to
   boil in; [=a]l burning + f[ae]t vat.]
   A caldron of boiling water into which an accused person
   plunged his forearm as a test of innocence or guilt.
   [1913 Webster]

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

ALFET, obsolete. A vessel in which hot water was put, for the purpose of
dipping a criminal's arm in it up to the elbow.