The Collaborative International Dictionary of English v.0.48:

Seizer \Seiz"er\, n.
   One who, or that which, seizes.
   [1913 Webster]

WordNet (r) 3.0 (2006):

seizer
    n 1: a kidnapper who drugs men and takes them for compulsory
         service aboard a ship [syn: shanghaier, seizer]