Acedia is the "despair from weakness" which Kierkegaard analysed as the "despairing refusal to be oneself."

Metaphysically and theologically, the notion of acedia means that a man does not, in the last resort, give the consent of his will to his own being; that beneath the dynamic activity of his existence, he is still not at one with himself; that, as the Middle Ages expressed it, sadness overwhelms him when he is confronted with the divine goodness immanent in himself.

No, the contrary of acedia is not the spirit of work in the sense of the work of every day, of earning one's living; it is man's happy and cheerful affirmation of his own being, his acquiescence in the world and in God -which is to say love. Love that certainly brings a particular freshness and readiness to work along with it, but that no one with the least experience could conceivably confuse with the tense activity of the fanatical "worker." 

Acedia was reckoned among the vitia capitalia, as one of the seven capital or cardinal sins, for they were not called "capital" because of the best known rendering of caput; caput certainly means "head," but it also means "source" or "spring"-and that is the meaning in this case. They are sins from which other faults follow "naturally," one is tempted to say, as from a source. Idleness-and this is how we get back to the question-idleness, according to traditional teaching, is the source of many faults and among others of that deep-seated lack of calm which makes leisure impossible. 

Among other faults, certainly, and one of the children of acedia, is despair, which amounts to saying that despair and the incapacity for leisure are twins-a revealing thought that explains, among other things, the hidden meaning of that very questionable saying, "work and don't despair." Idleness, in the old sense of the word, so far from being synonymous with leisure, is more nearly the inner prerequisite which renders leisure impossible : it might be described as the utter absence of leisure, or the very opposite of leisure.

Josef Pieper
Leisure: The Basis of Culture