Sloth is "sorrow in the face of spiritual good inasmuch as it is God's good" (St. Thomas Aquinas). Sloth is not just laziness, but especially spiritual laziness. Sins which stem from the vice of sloth include lukewarmness toward divine precepts, drifting to what is forbidden and frequenting the occasion of sin, faintheartedness and despair of salvation. The remedy for sloth is remembering one's promised eternal reward as well as one's punishment for sin. Archbishop Fulton Sheen taught, "Sloth is a malady of the will which causes us to neglect our duties. Sloth may be either physical or spiritual. It is physical when it manifests itself in laziness, procrastination, idleness, indifference and nonchalance. It is spiritual when it shows itself in an indifference to character betterment, a distaste for the spiritual, a hurried crowding of devotions, a lukewarmness and failure to cultivate new virtue" (Victory over Vice, p. 73).
Source: Saunders, Rev. William 3