Whoa interj. Stop; stand; hold.  See Ho, 2.
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  Ho, Hoa  interj.
  1. Halloo! attend! -- a call to excite attention, or to give notice of approach. “What noise there, ho?”    “Ho! who's within?”  --Shak.
  2.  Stop! stand still! hold! -- a word now used by teamsters, but formerly to order the cessation of anything. [Written also whoa, and, formerly, hoo.]
     The duke . . . pulled out his sword and cried =\“Hoo!”\=   --Chaucer.
     An herald on a scaffold made an hoo.   --Chaucer.
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