Seven Deadly Sins: Sloth

1.
Sloth: From the Middle English slowthe, 'sloth'; the Old English slaw, the Old Saxon sleu and the Old High German sleo, 'slow', 'dull' or 'blunt'; and perhaps allied to the Latin laevus and the Greek laios, 'the left', and the Sanskrit srevayati, 'he causes to fail'. In Matthew 25:26, Jesus uses the term okneros, 'shrinking' or 'hesitating', to refer, in the parable of the talents, to the man who hid his portion under the ground out of fear. There are two other references to sloth in the Epistles. (Among Catholic writers, the Late Latin Aquinan term acedia, 'sloth', is sometimes preferred to the Saxon term. Acedia stems directly from the Greek akedos, 'careless', from a, 'not', and kedos, 'care', 'grief' and 'anxiety', derived from the Avestan sadra, 'sorrow'.)

2.
”To be spiritually lazy.”
Like the animal that shares its name, sloth is lazy, but there's more to it than that: it's a spiritual laziness. Before the list of sins to avoid was narrowed down to the seven we know today, there was acedia, the listlessness that afflicted monks around the middle of the day, and tristitia, or sadness. The two were combined into sloth by Pope Gregory the Great in the sixth century.
Sloth was considered a sin because it dissuaded people from their spiritual duties: helping others, going to church and achieving salvation. (The virtue against which sloth is said to sin is zeal.) To lose one's faith in God would be sinful indeed, and this is what one risked by being slothful.
In early medieval literature, sloth was often depicted as a person, sometimes a fat one, riding an ass (a slow, plodding beast); in Dante's Inferno, the slothful (or "sullen") are forever submerged in the Styx, their sighs creating ripples on its surface. This is an apt image, as sloth is often attributed to depression, and many who whose suffer from it describe feeling as though they are underwater.
It's probably fair to say that sloth is viewed today as the most benign of character flaws among the Seven Deadly Sins, and even in modern church teachings, sloth becomes a mortal sin only when it's deliberately (rather than inadvertently) committed by refusing to maintain a relationship with God.