Lethargy came to English via Old French from late Latin "lethargia," from Greek "lethargia," from  "lethargos," (forgetful), from the base of "lanthanesthai" (forget). 

The Oxford English Dictionary (OED) first records the noun "lethargy" in English in the early 1600s. Specifically, the earliest evidence the OED has is from 1608, in the writings of William Shakespeare. The word "lethargy" also appears as a noun in Middle English, between 1150-1500.  According to the OED, the adjective "lethargic" first appeared in the English language in the 14th century. 

Examples of "lethargy" in a sentence: 
"I snapped out of my lethargy and began cleaning the house." 
"The symptoms are extreme tiredness, lethargy." 
"Symptoms include sore eyes, lethargy, headaches and skin irritations." 
"Sometimes its departure can be slow, a diminishing of the pain and the lethargy over a few days." 
"But a strange lethargy has kept him in his room for most of the time since he was released." 
"Please suggest some music to battle this lethargy." 
"The results showed that workers whose backs were pushed into constant motion experienced less back pain and lethargy." 
"The thought of cooking anything fills you with lethargy, yet your newly enlarged belly makes you feel strangely hungry." 

Just the facts:

LETHARGY Definition & Meaning - Merriam-Webster