The Collaborative International Dictionary of English v.0.48:

Significavit \Sig`ni*fi*ca"vit\, n. [L., (he) has signified,
   perf. ind. of significare to signify.] (Eng. Eccl. Law)
   Formerly, a writ issuing out of chancery, upon certificate
   given by the ordinary, of a man's standing excommunicate by
   the space of forty days, for the laying him up in prison till
   he submit himself to the authority of the church. --Crabb.
   [1913 Webster]

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

SIGNIFICAVIT, eccl. law. When this word is used alone, it means the bishop's
certificate to the court of chancery, in order to obtain the writ of
excommunication; but where the words writ of significavit are used, the
meaning is the same as writ de excommunicato capiendo. 2 Burn's Eccl. L.
248; Shelf. on Mar. & Div. 502.