The historian Procopius of Caesarea lived during the reign of the Roman emperor Justinian (482-565). His history of the wars of Justinian was published during his lifetime, but another of his works, now referred to as The Secret History, remained unpublished until the manuscript was discovered in the Vatican Library and published in 1623.
Although unpublished for centuries, the existence of The Secret History was known earlier, because it is mentioned in a 10th century encyclopedia called the Suda; there, the work is referred to by the Greek word Anekdota, which in Latin is Anecdota, “unpublished writings.” A very good reason that the work was not published in the time of Justinian is the fact that it contains extremely unflattering stories about the private lives of the emperor and his wife.
The earliest meaning of anecdote in English is “Secret, private, or hitherto unpublished narratives or details of history.” Later, the word came to have its present meaning: “The narrative of a detached incident, or of a single event, told as being in itself interesting or striking.”
As Vice-President, Coolidge and his vivacious wife Grace were invited to quite a few parties, where the legend of “Silent Cal” was born. It is from this time that most of the jokes and anecdotes involving Coolidge originate.
The adjective anecdotal dates from the 18th century. It can mean simply “pertaining to anecdotes,” but in modern usage it is often used in the sense of “unreliable.”
anecdotal: based on or consisting of reports or observations of usually unscientific observers –Merriam-Webster Unabridged
anecdotal: (of an account) not necessarily true or reliable, because based on personal accounts rather than facts or research. –OxfordDictionaries
The legal term hearsay refers to “anecdotal evidence/testimony”:
hearsay: second-hand evidence in which the witness is not telling what he/she knows personally, but what others have said to him/her. –Legal Dictionary at law.com
Examples of the use of anecdotal and anecdotally:
I hope you realize that you’re using the same rhetoric that the people who don’t believe in climate change use. You’re using anecdotal warmth in a particular part of the globe to prove climate change.
The medical community has soundly refuted these theories [that autism is caused by vaccines], but a very passionate group of parents and researchers continue to disagree, based on anecdotal evidence.
The recent medical controversy over whether vaccinations cause autism reveals a habit of human cognition—thinking anecdotally comes naturally, whereas thinking scientifically does not.