Answer: The sinfulness of that estate whereinto man fell, consists in the guilt of Adam’s first sin₁, the want of that righteousness wherein he was created, and the corruption of his nature, whereby he is utterly indisposed, disabled, and made opposite unto all that is spiritually good, and wholly inclined to all evil, and that continually₂; which is commonly called original sin, and from which do proceed all actual transgressions.₃

1. Romans 5:12,19
2. Romans 3:10-19; Ephesians 2:1-3; Romans 5:6; Romans 8:7-8; Genesis 6:5
3. James 1:14-15; Matthew 15:19

For further study see:

1. WSC 18: Wherein consists the sinfulness of that estate whereinto man fell?
2. Elder Gregory Moeck of First Orthodox Presbyterian Church, San Francisco on WLC 25a: “Total Depravity is not a material wickedness, but a want of original righteousness”
   
3. WLC 25c – Does total depravity affect all aspects of man in the same way?
   
4. WLC 25d – How does the depravity of man work?