Easton’s Bible Dictionary
Dog: 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.