weary

Lassitude and weariness make an interesting pair. As with many nearly synonymous pairs of words in English, one is derived from Latin and the other from Old English. Even though they both mean “the condition of being tired,” they are used in different ways. Following a common pattern, the Latinate word tends to be used in technical, medical, and formal writing, and the Old English-derived word is used when referring to physical, emotional, and spiritual qualities.

Lassitude comes from the Latin word lassus, meaning “weary.” Our English spelling comes from the French word that developed directly from Latin, borrowed in the 15th century. In French, the word las (masculine) or lasse (feminine) means “weary” or “tired,” and the idiom être las de means “to be sick and tired of.” This led to another English word with the same root: alas, a word that expresses sadness or disappointment, but conveys some measure of fatigue and resignation as well.

Though it sometimes is just a fancy word for fatigue in medical contexts, lassitude is also used in ways that are metaphorical and closer in meaning to “negligence”:

Congress was being choked by pettiness and lassitude.

The case was delayed because of sheer lassitude.

The failure was the result of moral lassitude.