The word "choler" comes from the ancient Greek word khlōros, meaning "pale green" or "greenish-yellow".  It entered Old French as colere and then Late Latin as cholera.  A predominance of choler was associated with a quick-tempered, irritable, and ambitious personality. This is why "choler" is now used to mean anger or irritability.  The word "choler" is primarily a noun, As a noun, which refers to anger, wrath, or irritability, and historically, it was also used to describe yellow bile.  It can also function as an adjective in older forms, such as "choleric", a term that was used to describe someone who is easily angered or has a bad-tempered disposition. 

The term Cholera came to refer to a severe acute diarrheal infection caused by the Vibrio cholerae bacterium, which contaminates food and water.   The infection results in profuse, painless, watery diarrhea and can lead to rapid and severe dehydration and death if not treated with prompt fluid and electrolyte replacement. 

According to the Oxford English Dictionary (OED), the word "choler" was first used in Middle English before 1382. The OED's earliest evidence for the term comes from the Bible (Wycliffite, early version). 

Examples of "choler" used in a sentence:  
"Emerson's fiery complexion subsided by gradual degrees and his acute intelligence triumphed over his choler."  
"The medieval notion of humors suggested that too much choler, or bile, could make a person quick-tempered."  
"His reputation for choler made employees reluctant to ask him questions."  
"The big head, bright eyes, and steely mouth suggest brains, pride, and choler."

1 hour ago, drpaladin said:

Through the humours and bile, choler is related to bilious.

Now choler as word of the day has highlighted a specific use and meaning of the word humour; a use that has nothing to do with being funny, no matter how much we might joke about it.

Just the facts:

CHOLER Definition & Meaning - Merriam-Webster

CHOLER Synonyms: 153 Similar and Opposite Words | Merriam-Webster Thesaurus