Lethargy, languor, lassitude, stupor, torpor, torpidity are comparable when meaning physical and mental inertness.
Lethargy implies a state marked by an aversion to activity which may be constitutional but is typically induced by disease, extreme fatigue or exhaustion, overeating or overdrinking, or constant frustration and which exhibits itself in drowsiness or apathy.
Languor (compare LANGUID ) has nearly lost its basic application to a condition of weakness, faintness, or delicacy of constitution induced by illness and serving as a bar to exertion or effort and has come to imply an inertia such as results from soft living, from an enervating climate, or from amorous emotion.
Lassitude implies such a listless seedy mental or physical condition as may result from strain, overwork, poor health, or intense worry; it usually connotes an inertia of mind or body which one has not the strength to fight.
Stupor implies a state of heaviness when the mind is deadened (as by extreme drowsiness, intoxication, narcotic poisoning, or the coma of illness); the term may imply any state from a dreamy trancelike condition to almost complete unconsciousness.
Torpor and torpidity basically suggest the condition of a hibernating animal which has lost all power of exertion or of feeling. Both terms, especially when employed in reference to persons, usually imply extreme sluggishness and inertness (as in some forms of insanity); torpidity, however, probably more often applies to a physical condition and torpor to a mental state.