More than other sins, the definition of sloth has changed considerably since its original inclusion among the seven deadly sins. In fact it was first called the sin of sadness or despair. It had been in the early years of Christianity characterized by what modern writers would now describe as melancholy: apathy, depression, and joylessness the last being viewed as being a refusal to enjoy the goodness of God and the world he created. Originally, its place was fulfilled by two other aspects, acedia and sadness. The former described a spiritual apathy that affected the faithful by discouraging them from their religious work. Sadness (tristitia in Latin) described a feeling of dissatisfaction or discontent, which caused unhappiness with one's current situation. When Thomas Aquinas selected acedia for his list, he described it as an "uneasiness of the mind", being a progenitor for lesser sins such as restlessness and instability. Dante refined this definition further, describing sloth as being the "failure to love God with all one's heart, all one's mind and all one's soul." He also described it as the middle sin, and as such was the only sin characterised by an absence or insufficiency of love. In his "Purgatorio", the slothful penitents were made to run continuously at top speed.
The modern view of the vice, as highlighted by its contrary virtue of zeal or diligence, is that it represents the failure to utilize one's talents and gifts. For example, a student who does not work beyond what is required (and thus fails to achieve his or her full potential) could be labeled slothful.
Current interpretations are therefore much less stringent and comprehensive than they were in medieval times, and portray sloth as being more simply a sin of laziness or indifference, of an unwillingness to act, an unwillingness to care (rather than a failure to love God and his works). For this reason sloth is now often seen as being considerably less serious than the other sins, more a sin of omission than of commission. The South American animal was named after this sin by Roman Catholic explorers. See also <3 Wrath Vanity Envy Lust Gluttony Greed
Actually, Sloth was originally "Acedia" and "Tristia", both Latin words. All the sins were originally written in Latin, however, Sloth was the only sin given two Latin words to describe it. As a Latin student myself, i should inform you that though "tristia" does mean saddness, "Acedia" means carelessness, which dante transversed to laziness.