The Collaborative International Dictionary of English v.0.48:
Holloa \Hol*loa"\, interj., n. & v. i. Same as Hollo. [1913 Webster]WordNet (r) 3.0 (2006):
holloa n 1: a very loud utterance (like the sound of an animal); "his bellow filled the hallway" [syn: bellow, bellowing, holla, holler, hollering, hollo, holloa, roar, roaring, yowl]