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Webster 1913 Edition

(hăl-lō′)

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halloa

A loud exclamation; a call to invite attention to something or to incite; a shout.
- 1903, Sir Arthur Conan Doyle, The Adventure of the Solitary Cyclist
  "Halloa! Stop there!" he shouted, holding his bicycle to block our road.

halloa ‎(third-person singular simple present halloas, present participle halloaing, simple past and past participle halloaed)