It isn't sadness; that is the biggest misconception. People treat it like an emotion infecting a blue day, labeling slightly soaked cheeks as this ailment of the mind. The term is cracked like a whip in stinging insult: weak, powerless, loser, outcast.
It is feeling a lack of feeling, where one exists in a mental state of wanting to be anything but lethargic yet finding nothing worthwhile inside with which to take action: no talent, no skill, no interest.
It is not only not believing one has any energy but seeing nothing to which to give it, in yourself, in others, in the world.
It is severe despondency and dejection, consuming worlds like oozing, viscose magma dribbling uncontrollably as burning ***** from the mountain's fiery mouth burping filthily as is sludges onward.
It isn't sorrow, or misery, or despair.
It is inadequacy, an ebb of interest in life, with a sliver of interest to take it.