Douglas Harper's Etymology Dictionary
"servile black man," 1922, somewhat inaccurately in reference to the humble, pious, but strong-willed main character in Harriet Beecher Stowe's novel, "Uncle Tom's Cabin" (1852). The image implied in the insult perhaps is more traceable to the late 19c. minstel show versions of the story, which reached a far wider audience than the book.\n\nI don't recall anyone in the 1920s using the term 'Uncle Tom' as an epithet. But what's amazing is how fast it caught on (in the 1930s). Black scholars picked up (the term) and just started throwing it at each other.
[Ernest Allen, quoted in Hamilton, Kendra, "The Strange Career of Uncle Tom," Black Issues in Higher Education, June 2002]
\nAs a verb, attested from 1937.Wikipedia
Uncle Tom is the title character of Harriet Beecher Stowe's 1852 novel, Uncle Tom's Cabin. The negative epithet is the result of later works derived from the original novel.
Usage examples of "uncle tom".
Adolf Hitler was on top of the list, which also included murdered American Nazi leader George Lincoln Rockwell, Martin Luther King assassin James Earl Ray, Confederate cavalry General Nathan Bedford Forrest, and one fictional character: slave overseer Simon Legree from Uncle Tom's Cabin.
A few minutes brought them to the window of Uncle Tom's cottage, and Eliza, stopping, tapped lightly on the window-pane.
It was Uncle Tom, who had come in, and stood listening to the conversation at the door.
The incorruptible fidelity, piety and honesty, of Uncle Tom, had more than one development, to her personal knowledge.