The etymology of dolor comes directly from the Latin word dolor, which means "pain," "grief," or "sorrow".  This Latin root is the source for English words like dolour and dolorous.  The Latin term was adopted into other languages. For instance, in Spanish, it forms the root of the word dolor (pain) and is used in religious contexts like Maria de los Dolores ("Mary of the Sorrows").  The word's origin can be traced back to the Proto-Indo-European root delh-, meaning "to chop".  The word dolor can now mean: mental anguish, pain, sorrow, sadness, or grief. 

According to the Oxford English Dictionary (OED), the noun dolor was first used in English during the Middle English period (1150-1500) when it appeared in the text Seven Sages, around 1320.  The word is a borrowing from French (dolour), which traces back to the Latin word dolor. 

Shakespeare also used the word in some of his works, such as in Macbeth.  "Each new morn / New widows howl, new orphans cry, new sorrows / Strike heaven on the face, that it resounds / As if it felt with Scotland and yelled out / Like syllable of dolor." (written and performed between 1606 and 1607) 

***Don't confused the word "dolor" with yesterday's word "choler".*** 

Examples of "dolor" used in a sentence:   
"Her sad poems grew out of a deep dolor that lasted for months,"  (expressing sorrow) 
"Your sad-eyed dog might express a great deal of dolor when you leave him alone for the day." (referring to sadness or pain) 
"For me the great shame and dolor of our times is the story of immigrant children." (expressing sorrow or sadness) 
"All elements in this rainbow program...are carefully contrived and guaranteed to lift the dolors of winter and put you in a buttercup mood." (referring to the sorrows or pains of winter)  
"In three hours of listless dolor, Matt Reeves’s oppressively dour 'The Batman', which came out this spring, turned its hero into a comically emo Bat-adolescent." (expressing a sad mood) 

Just the facts:

DOLOR Definition & Meaning - Merriam-Webster

DOLOR Synonyms: 98 Similar and Opposite Words | Merriam-Webster Thesaurus