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)
“Sloth is the condition in which a man is fully aware of the proper means of his salvation and refuses to take them because the whole apparatus of salvation fills him with tedium and disgust.”