Green’s Dictionary of Slang

ware (the) hawk! excl.

also ware the bull! war hawk! warrocks!
[hunting jargon ware hawk!, a warning cry either to or of animals. The hawk personifies any ‘grasping’ person, whether working for or against the law]

[17C–1900s] a warning cry, orig. indicating that a bailiff or constable is approaching; thus war-hawk n., a bailiff.