Uncle Tom is defined as an ethnic slur directed at an African American man who appears to be acting especially servile or submissive towards Caucasians. The expression comes from a novel by Harriet Beecher Stowe called Uncle Tom's Cabin in which Tom was a black man who showed great deference and obedience to whites.(noun)
An example of Uncle Tom is an insult addressed towards a black man who seems too quick to do things to please white men.
See Uncle Tom in Webster's New World College Dictionary
Origin: after the main character, an elderly black slave, in Harriet Beecher Stowe's antislavery novel, Uncle Tom's Cabin (1852)
See Uncle Tom in American Heritage Dictionary 4
Origin: After Uncle Tom, a character in Uncle Tom's Cabin, a novel by Harriet Beecher Stowe.