Green’s Dictionary of Slang

Hays n.

[Hays, the name of a High Constable at the time]

used as a warning among thieves, meaning that they should run away.

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[US]‘Ned Buntline’ Mysteries and Miseries of N.Y. I 12: Cut and run, my darling! Hays is the word, and off you go! [Ibid.] IV 38: ‘Hays!’ cried the crowd. ‘Hays, Mose, you’ve done him!’ and the b’hoys made off at full speed.