Merriam-Webster Collegiate Dictionary - hollo

 I. verb  or halloo; also hallo  Date: 14th century  intransitive verb to cry ~ ; holler  transitive verb  1. to call or cry ~ to  2. to utter loudly ; holler  II. noun  or halloo; also hallo  (plural ~s or halloos; also hallos)  Date: 15th century an exclamation or call of ~  III. interjection  or halloo; also hallo  Etymology: origin unknown  Date: 1588  1. — used to attract attention (as when a fox is spied during a fox hunt)  2. — used as a call of encouragement or jubilation