The cry made by huntsmen when they uncouple the dogs in hunting the hare. Also to pointers and setters when they make a point. Tally-ho! (q.v.) is the cry when a fox breaks cover. So! or see! is to call attention, and ho! is virtually “hie after him.”
Now is the fox drevin to hole. Hoo to hym! Hoo! Hoo! For and he acpe out he will you alle undo.
Excerpta Historica, p. 279.
“If ye bounte at the hare, ye shall say, atte uncoupling, hors de couple, avaunt! And after, three times, Sohow! Sohow!” - A fifteenth-century translation of Reliquæ Antiquæ.
“When a stag breaks covert the cry is `tayho!' ... when a hare ... `soho!' ” —Herbert: Field Sports, vol. iii. appendix B. p. 313.
Of course “Ho!” is often used merely to call attention. Thus we say to one in advance, “Ho! stop!” and “Ho! every one that thirsteth, come ye to the waters” (Isaiah lv. 1). This use of the word is a contracted form of haloo! In the hunting-field “So-ho” is doubtless a cry to encourage the dogs to follow up the quarry.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894