What is Admonitio Trina?

Legal Definition
A triple cr threefold warning, given, in old times, to a prisoner standing mute, before he was subjected to the peine forte et dure. 4 BL Comm. 325 ; 4 Steph. Comm. 391.
-- Black's Law Dictionary
Legal Definition
A warning which was given to a prisoner standing mute.
-- Ballentine's Law Dictionary