The Collaborative International Dictionary of English v.0.48:

Shoo \Shoo\, interj. [Cf. G. scheuchen to scare, drive away.]
   Begone; away; -- an expression used in frightening away
   animals, especially fowls.
   [1913 Webster]
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

shoo
    v 1: drive away by crying `shoo!' [syn: shoo off, shoo,
         shoo away]