Source : Webster's Revised Unabridged Dictionary (1913)

Halloo \Hal*loo"\, n. [Perh. fr. ah + lo; cf. AS. eal[=a], G.
   halloh, F. haler to set (a dog) on. Cf. {Hollo}, interj.]
   A loud exclamation; a call to invite attention or to incite a
   person or an animal; a shout.

         List! List! I hear Some far off halloo break the silent
         air.                                     --Milton.

Halloo \Hal*loo"\, v. i. [imp. & p. p. {Hallooed}; p. pr. & vb.
   n. {Halloing}.]
   To cry out; to exclaim with a loud voice; to call to a
   person, as by the word halloo.

         Country folks hallooed and hooted after me. --Sir P.
                                                  Sidney.

Halloo \Hal*loo"\, v. t.
   1. To encourage with shouts.

            Old John hallooes his hounds again.   --Prior.

   2. To chase with shouts or outcries.

            If I fly . . . Halloo me like a hare. --Shak.

   3. To call or shout to; to hail. --Shak.

Halloo \Hal*loo"\, interj. [OE. halow. See {Halloo}, n.]
   An exclamation to call attention or to encourage one.

Source : WordNet®

halloo
     n : a shout to attract attention; "he gave a great halloo but no
         one heard him"
     v 1: urge on with shouts; "halloo the dogs in a hunt"
     2: shout `halloo', as when greeting someone or attracting
        attention