The Collaborative International Dictionary of English v.0.48:

Obreption \Ob*rep"tion\, n. [L. obreptio, fr. obrepere,
   obreptum, to creep up to; ob (see Ob-) + repere to creep.]
   1. The act of creeping upon with secrecy or by surprise.
      [Obs.] --Cudworth.
      [1913 Webster]

   2. (Scots Law) The obtaining gifts of escheat by fraud or
      surprise. --Bell.
      [1913 Webster]

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

OBREPTION, civil law. Surprise. Dig. 3,5,8,1. Vide Surprise.