I came across this sentence: "Jim and Huck spent days of indolent lassitude on the craft." I wasn't quite sure what to make of the phrase 'indolent lassitude' because to my mind they both sort of signified the same thing.
The two words, indolent and lassitude, are close to being synonyms, but they are not.
Indolent is an adjective which means, in essence, lazy. The noun form is indolence (i.e., laziness).
Lassitude, on the other hand, could be linked to laziness, but it could also be linked to weariness, diminished energy, listlessness, or enervation.
In other words, if you, in a wearied state, were to lie down, you might just slip into a state of indolence as well. You may then feel your laziness to be justified or deserved and would not likely feel guilty.
People normally consider laziness (or sloth) to be a negative thing--one of the "seven deadly sins," but if it is the result of being in an enervated, weakened state, people find it to be at least understandable, if not excusable.