"Power tends to corrupt, and absolute power corrupts absolutely. Great men are almost always bad men."
Meaning
Literal meaning.
Origin
This arose as a quotation by John Emerich Edward Dalberg Acton, first Baron Acton (1834–1902). The historian and moralist, who was otherwise known simply as Lord Acton, expressed this opinion in a letter to Bishop Mandell Creighton in 1887:
"Power tends to corrupt, and absolute power corrupts absolutely. Great men are almost always bad men."
Another English politician with no shortage of names - William Pitt, the Elder, The Earl of Chatham and British Prime Minister from 1766 to 1778, is sometimes wrongly attributed as the source. He did say something similar, in a speech to the UK House of Lords in 1770:
"Unlimited power is apt to corrupt the minds of those who possess it"