Diligent, which entered English in the 14th century by way of Anglo-French, comes from the Latin diligere, which means "to value highly, take delight in," but in English it has always meant careful and hard-working.

The earliest known use of the word "diligent" in English is from the Middle English period (1150-1500), with the Oxford English Dictionary (OED) citing evidence from 1340 in "Avenbite of Inwyt." 

Using "diligent" in a sentence: 
"Their lawyer was extremely diligent in preparing their case, which was done in a careful and detailed way."
"The discovery was made after years of diligent research." 
"He was diligent in the matter of temperature control." 
"His range of reading was extensive, and he was a most diligent scholar and writer." 
"From his youth he was diligent in his studies and a great reader, and during his college life showed a marked talent for extemporaneous speaking." 
"There was a diligent inquiry and now most believe they acted on correspondence produced by Henry Woods on his own lawyer's request."

Just the facts:

DILIGENT Definition & Meaning - Merriam-Webster