Wiktionary
n. 1 Any dog bred; trained or used for the purpose of attacking a target, either on command or on sight; trained either as a specialized form of police dog or a security force dog. 2 A dog used in dogfighting; a bloodsport. 3 A person who engages in rhetorical attacks on behalf of another.
WordNet
n. a watchdog trained to attack on command
Wikipedia
Usage examples of "attack dog".
Yet the rabid mad dog Left, using the yapper Charles Schumer as their lead attack dog, viciously assailed him as unqualified.
A trained attack dog, German shepherd, weighing eighty or ninety pounds.
Like a trained attack dog watching an intruder blunder nearer to the point of no return.
He sounded like the most vicious attack dog ever bred, not a sweet black Lab.
You'd have a better chance against a black belt in karate than an attack dog.
Jason fired, first one cartridge and then the second, and as the darts were embedded, he whipped his left arm around the attack dog’.
The Attack Dog Trainer was leaning so far forward, he was in danger of toppling over and splitting Pretorius's desk with the blade of his face.
The cliffs were one place where she might face an attack dog and win.
He's not an attack dog but he's big and he'll make a bushel of noise if someone was to come by uninvited.