The Collaborative International Dictionary of English v.0.48:

Plagium \Pla"gi*um\, n. [L.] (Civil Law)
   Manstealing; kidnaping.
   [1913 Webster]

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

PLAGIUM. Man stealing, kidnapping. This offence is the crimen plagii of the
Romans. Alis. Pr. Cr. Law, 280, 281.