Frequently mentioned both in the Old and New Testaments. Dogs were used by the Hebrews as a watch for their houses (Isaiah 56:10), and for guarding their flocks (Job 30:1). There were also then as now troops of semi-wild dogs that wandered about devouring dead bodies and the offal of the streets (1 Kings 14:11; 16:4; 21:19, 23; 22:38; Psalm 59:6, 14).

As the dog was an unclean animal, the terms "dog," "dog's head," "dead dog," were used as terms of reproach or of humiliation (1 Samuel 24:14; 2 Samuel 3:8; 9:8; 16:9). Paul calls false apostles "dogs" (Philippians 3:2). Those who are shut out of the kingdom of heaven are also so designated (Revelation 22:15). Persecutors are called "dogs" (Psalm 22:16). Hazael's words, "Thy servant which is but a dog" (2 Kings 8:13), are spoken in mock humility=impossible that one so contemptible as he should attain to such power.

Noah Webster's New International Dictionary of the English Language

1. (n.) A quadruped of the genus Canis, esp. the domestic dog (C. familiaris).

2. (n.) A mean, worthless fellow; a wretch.

3. (n.) A fellow; -- used humorously or contemptuously; as, a sly dog; a lazy dog.

4. (n.) One of the two constellations, Canis Major and Canis Minor, or the Greater Dog and the Lesser Dog. Canis Major contains the Dog Star (Sirius).

5. (n.) An iron for holding wood in a fireplace; a firedog; an andiron.

6. (n.) A grappling iron, with a claw or claws, for fastening into wood or other heavy articles, for the purpose of raising or moving them.

7. (n.) An iron with fangs fastening a log in a saw pit, or on the carriage of a sawmill.

8. (n.) A piece in machinery acting as a catch or clutch; especially, the carrier of a lathe, also, an adjustable stop to change motion, as in a machine tool.

9. (v. t.) To hunt or track like a hound; to follow insidiously or indefatigably; to chase with a dog or dogs; to worry, as if by dogs; to hound with importunity.