What is Burning In The Hand?

Legal Definition
In old English criminal law, laymen, upon being accorded the benefit of clergy, were burned with a hot iron in the brawn of the left thumb, in order that, being thus marked, they could not again claim their clergy. 4 Bl. Comm. 367.
-- Black's Law Dictionary
Legal Definition
A practice of branding to prevent a second claiming of benefit of clergy.
-- Ballentine's Law Dictionary