Definition and Usage:
Sackcloth and dust are symbols of mourning, repentance, and humility in the Bible. Sackcloth is a coarse, rough fabric, typically made from goat's hair, worn as a sign of penitence or grief. Dust, often associated with ashes, signifies mortality and contrition. Together, they represent a physical manifestation of inner sorrow and repentance before God.

Biblical References:

1. Mourning and Grief:
Sackcloth is frequently mentioned in the context of mourning. In Genesis 37:34, Jacob mourns for his son Joseph, believing him to be dead: "Then Jacob tore his clothes, put on sackcloth, and mourned for his son many days." This act of wearing sackcloth signifies deep personal loss and sorrow.

2. Repentance and Humility:
Sackcloth and dust are also used to express repentance and humility before God. In the book of Jonah, the people of Nineveh, upon hearing Jonah's warning of impending judgment, respond with fasting and donning sackcloth: "When Jonah’s warning reached the king of Nineveh, he rose from his throne, took off his royal robes, covered himself with sackcloth, and sat in the dust" (Jonah 3:6). This act of humility and repentance leads to God's mercy on the city.

3. Prophetic Symbolism:
Prophets often used sackcloth as a symbol of impending judgment or as a call to repentance. Isaiah 20:2-3 describes the prophet Isaiah walking barefoot and naked, a symbolic act that included wearing sackcloth, to illustrate the coming judgment on Egypt and Cush.

4. Expressions of Desperation:
In times of national crisis or personal desperation, sackcloth and dust are used to appeal to God for intervention. In Esther 4:1, Mordecai learns of Haman's plot to destroy the Jews and responds by tearing his clothes, putting on sackcloth and ashes, and going out into the city, wailing loudly and bitterly.

5. Lamentation and Intercession:
Sackcloth is also associated with lamentation and intercession. In Daniel 9:3, Daniel turns to God in prayer and supplication with fasting, sackcloth, and ashes, seeking forgiveness for the sins of Israel: "So I turned my attention to the Lord God to seek Him by prayer and petition, with fasting, sackcloth, and ashes."

Cultural and Historical Context:
In ancient Near Eastern cultures, sackcloth was a common garment for the poor and was used in times of mourning and distress. The act of sitting in dust or ashes was a way to physically demonstrate one's lowliness and submission to God. These practices were deeply ingrained in the cultural expressions of grief and repentance.

Theological Significance:
The use of sackcloth and dust in the Bible underscores the importance of humility and repentance in the believer's relationship with God. It serves as a reminder of human frailty and the need for divine mercy. The physical acts of wearing sackcloth and sitting in dust are outward expressions of an inward reality—a heart contrite and broken before the Lord.

Conclusion:
Sackcloth and dust remain powerful symbols within the biblical narrative, illustrating the depth of human sorrow and the earnestness of repentance. They call believers to a posture of humility, recognizing their dependence on God's grace and forgiveness.