Most people who say "Machiavellian" have never read Machiavelli, and for a long time neither had the English who first used the word.
Niccolò Machiavelli was a Florentine statesman who lived from 1469 to 1527. He wrote The Prince around 1513, and it was not published until 1532, five years after his death. The book advises rulers to put advantage ahead of conventional morality. Many early readers were shocked, and Britannica notes that it earned him a reputation as an atheist and immoral cynic.
The adjective reached English in the 1570s. That is strikingly early. Etymonline points out that the word was already an insult in English well before his works were available in the language. The Discourses appeared in English in 1636, and The Prince only in 1640.
So where did the English get their opinion? Etymonline credits two sources, in part. The Catholic Church placed his books on its Index of prohibited works in 1559. And French writers attacked him, notably the Huguenot Innocent Gentillet, whose Discourse against Machiavelli was published in 1576. It called his work the "Koran of the courtiers" and was translated into English in 1602.
English also had a shortened form of the name. From the 16th to the 18th century he was anglicized as "Machiavel." The derived noun Machiavellianism came later.
The fairness of the reputation is still argued. Rousseau and other Enlightenment writers took The Prince as a satire. Many modern scholars reject that, and Isaiah Berlin said he knew of nothing that read less like satire. Others see hidden meanings in the text.
What is not in doubt is the adjective's definition. Etymonline glosses it as cunning, deceitful, habitually duplicitous and destitute of political morality. It describes the man's caricature far better than his actual books.