Q: Wherein consists the sinfulness of that estate whereinto man fell?
A: The sinfulness of that estate whereinto man fell, consists in the guilt of Adam's first sin, the want of original righteousness, and the corruption of his whole nature, which is commonly called Original Sin; together with all actual transgressions which proceed from it.

Romans 5:12, 19. Wherefore, as by one man sin entered into the world, and death by sin; and so death passed upon all men, for that all have sinned... For as by one man's disobedience many were made sinners, so by the obedience of one shall many be made righteous.
1 Corinthians 15:22. For as in Adam all die, even so in Christ shall all be made alive.
Romans 5:6. For when we were yet without strength, in due time Christ died for the ungodly.
Ephesians 2:1-3. And you hath he quickened, who were dead in trespasses and sins; wherein in time past ye walked according to the course of this world, according to the prince of the power of the air, the spirit that now worketh in the children of disobedience: among whom also we all had our conversation in times past in the lusts of our flesh, fulfilling the desires of the flesh and of the mind; and were by nature the children of wrath, even as others.
Romans 8:7-8. Because the carnal mind is enmity against God: for it is not subject to the law of God, neither indeed can be. So then they that are in the flesh cannot please God.
Genesis 6:5. And God saw that the wickedness of man was great in the earth, and that every imagination of the thoughts of his heart was only evil continually.
Romans 3:10-20. As it is written, There is none righteous, no, not one: There is none that understandeth, there is none that seeketh after God. They are all gone out of the way, they are together become unprofitable; there is none that doeth good, no, not one. Their throat is an open sepulchre; with their tongues they have used deceit; the poison of asps is under their lips: Whose mouth is full of cursing and bitterness:Their feet are swift to shed blood: Destruction and misery are in their ways: And the way of peace have they not known: There is no fear of God before their eyes. Now we know that what things soever the law saith, it saith to them who are under the law: that every mouth may be stopped, and all the world may become guilty before God. Therefore by the deeds of the law there shall no flesh be justified in his sight: for by the law is the knowledge of sin.
Psalm 51:5. Behold, I was shapen in iniquity; and in sin did my mother conceive me.
Psalm 58:3. The wicked are estranged from the womb: they go astray as soon as they be born, speaking lies.
James 1:14-15. But every man is tempted, when he is drawn away of his own lust, and enticed. Then when lust hath conceived, it bringeth forth sin: and sin, when it is finished, bringeth forth death.
Matthew 15:19. For out of the heart proceed evil thoughts, murders, adulteries, fornications, thefts, false witness, blasphemies.

Westminster Project - See Andrew Wythe's Commentary on the Shorter Catechism for more detail