What it is
Sloth is the avoidance of physical or spiritual work.
Why you do it
You’re shiftless, lazy, and good fer nuthin’.
Your punishment in Hell will be
You’ll be thrown into snake pits.
In the Summa Theologiae, Medieval theologian Thomas Aquinas said Sloth is “sluggishness of the mind which neglects to begin good… [it] is evil in its effect, if it so oppresses man as to draw him away entirely from good deeds.” (2,35, ad 1)
The Pocket Catholic Catechism says:
Sloth is the desire for ease, even at the expense of doing the known will of God. Whatever we do in life requires effort. Everything we do is to be a means of salvation. The slothful person is unwilling to do what God wants because of the effort it takes to do it. Sloth becomes a sin when it slows down and even brings to a halt the energy we must expend in using the means to salvation.