The modern meaning of "sloth" is not what is meant by St. Thomas when he uses the word, although there is something of a relation between the two. He does not mean mere laziness or sluggishness - the sort of thing that might bring to our minds the animals called sloths. The vice known as sloth "is sadness about one's spiritual good, on account of the attendant bodily labor..." (ST I-II Q84 A4). Elsewhere he expands upon this:
Sloth, according to Damascene (De Fide Orth. ii, 14) is an oppressive sorrow, which, to wit, so weighs upon man's mind, that he wants to do nothing; thus acid things are also cold. Hence sloth implies a certain weariness of work, as appears from a gloss on Ps. 106:18, "Their soul abhorred all manner of meat," and from the definition of some who say that sloth is a "sluggishness of the mind which neglects to begin good" (ST II-II Q35 A1).So sloth isn't simple laziness - a vice in itself - but rather, it is a sorrow or sluggishness about doing good that prevents one from doing it. If a thief wants to quit stealing, but he thinks to himself that it would be too hard to give up his larcenous ways and get a real job to provide for his needs...that's sloth. I suppose the story from St. Bede I retold recently might be another example of this vice: the wicked man despaired of being able to repent...and so he didn't, to his own destruction.
And that very story gives us a good example of why we must be careful not to give into this sin. If it gets a grip upon us, we may very well find ourselves saying that the good we need to do in order to be reconciled to God is too much.