What is De Lunatico Inquirendo?

Legal Definition
The name of a writ directed to the sheriff, directing him to inquire by good and lawful men whether the party charged is, a lunatic or not. See 4 Rawle, 234; 1 Whart. 52; 5 Halst. 217; 6 Wend. 497.
-- Bouviers Law Dictionary
Legal Definition
The name of a writ directed to the sheriff, directing him to inquire by good and lawful men whether the party charged is a lunatic or not
-- Black's Law Dictionary
Legal Definition
A writ to have one’s sanity inquired into.
-- Ballentine's Law Dictionary