Pink slip refers to the American practice, by a human resources department, of including a discharge notice in an employee's pay envelope to notify the worker of his or her termination of employment or layoff. The "pink slip" has become a metonym for the termination of employment in general. According to an article in The New York Times, the editors of the Random House Dictionary have dated the term to at least as early as 1910.
The phrase most likely originated in vaudeville. When the United Booking Office (established in 1906) would issue a cancellation notice to an act, the notice was on a pink slip ("The Argot of Vaudeville Part I" New York Times, Dec. 16, 1917, p.X7.) Another possible etymology is that many applications (including termination papers) are done in triplicate form, with each copy on a different color of paper, one of which is typically pink.
In the Chilly Willy cartoon, "little Televillian", Mr. Stoop, tells Smedley that if he's disturbed while reading show scripts, he would be forced to pink slip Smedley, (which Smedley doesn't know the meaning of) and yells at him, " It means...you will be fired!".
In the UK and Ireland the equivalent of a pink slip is a P45; in Belgium the equivalent is known as a C4.