Objection 1. It would seem that original sin is not a habit. For original sin is the absence of original justice, as Anselm states (De Concep. Virg. ii, iii, xxvi), so that original sin is a privation. But privation is opposed to habit. Therefore original sin is not a habit.
Objection 2. Further, actual sin has the nature of fault more than original sin, in so far as it is more voluntary. Now the habit of actual sin has not the nature of a fault, else it would follow that a man while asleep, would be guilty of sin. Therefore no original habit has the nature of a fault.
Objection 3. Further, in wickedness act always precedes habit, because evil habits are not infused, but acquired. Now original sin is not preceded by an act. Therefore original sin is not a habit.
On the contrary, Augustine says in his book on the Baptism of infants (De Pecc. Merit. et Remiss. i, 39) that on account of original sin little children have the aptitude of concupiscence though they have not the act. Now aptitude denotes some kind of habit. Therefore original sin is a habit.
I answer that, As stated above (49, 4; 50, 1), habit is twofold. The first is a habit whereby power is inclined to an act: thus science and virtue are called habits. In this way original sin is not a habit. The second kind of habit is the disposition of a complex nature, whereby that nature is well or ill disposed to something, chiefly when such a disposition has become like a second nature, as in the case of sickness or health. In this sense original sin is a habit. For it is an inordinate disposition, arising from the destruction of the harmony which was essential to original justice, even as bodily sickness is an inordinate disposition of the body, by reason of the destruction of that equilibrium which is essential to health. Hence it is that original sin is called the "languor of nature" [Cf. Augustine, In Ps. 118, serm. iii].
Reply to Objection 1. As bodily sickness is partly a privation, in so far as it denotes the destruction of the equilibrium of health, and partly something positive, viz. the very humors that are inordinately disposed, so too original sin denotes the privation of original justice, and besides this, the inordinate disposition of the parts of the soul. Consequently it is not a pure privation, but a corrupt habit.
Reply to Objection 2. Actual sin is an inordinateness of an act: whereas original sin, being the sin of nature, is an inordinate disposition of nature, and has the character of fault through being transmitted from our first parent, as stated above (Question 81, Article 1). Now this inordinate disposition of nature is a kind of habit, whereas the inordinate disposition of an act is not: and for this reason original sin can be a habit, whereas actual sin cannot.
Reply to Objection 3. This objection considers the habit which inclines a power to an act: but original sin is not this kind of habit. Nevertheless a certain inclination to an inordinate act does follow from original sin, not directly, but indirectly, viz. by the removal of the obstacle, i.e. original justice, which hindered inordinate movements: just as an inclination to inordinate bodily movements results indirectly from bodily sickness. Nor is it necessary to says that original sin is a habit "infused," or a habit "acquired" (except by the act of our first parent, but not by our own act): but it is a habit "inborn" due to our corrupt origin.
Source: Summa Theologica (New Advent)