The word ignoramus comes right from the Latin word ignoramus, literally "we do not know," from the Latin root ignorare.  Ignoramus was a legal term in the 16th century that could be used during a trial when the prosecution presented insufficient evidence.  After George Ruggle's 1615 play "Ignoramus", a satirical comedy exposing lawyers' ignorance, it came into popular use to mean "fool" or "dunce." 

The word "ignoramus" was first used in English in the late 1500s, specifically in 1583, according to the Oxford English Dictionary (OED).  The word "ignoramus" appeared as a noun in the writings of Sir T. Smith in his 1583 work, De Republica Anglorum. 

Examples of "ignoramus" used in a sentence: 
"I'm a complete ignoramus where computers are concerned." 
"The reporter proved he was an ignoramus when he gave out false information during the broadcast." 
"Everyone knows the company president’s son is an ignoramus who will probably destroy the business once his father retires." 
"When our tour guide turned out to be an ignoramus who knew nothing about the city, we asked the company for a refund." 
"While Alan fancies himself a wine connoisseur, he is actually an ignoramus who cannot tell a chardonnay from a zinfandel."

Just the facts:

IGNORAMUS Definition & Meaning - Merriam-Webster