Anthrax lifted it from college newspapers...
Really? How do you know that?
Cuz I'm de man.

It was used by the Princeton Tiger starting in (ready for this?) the late 1800's.
Background info:
Princeton Tiger or Tiger Magazine is a college humor magazine published by Princeton University undergraduates since 1882. A number of its writers and editors later went on to notable literary careers, including Booth Tarkington, F. Scott Fitzgerald, and John McPhee.
The magazine's style has not remained stagnant over the past 120 years. While the format in the mid-20th century still tended towards humorous, light pieces, the off-campus circulation was broader and the writing reflected it. In recent years, it has increasingly focused on campus affairs, occasionally provoking complaints that the magazine is inscrutable to non-students, even alumni.
The March 30, 1893 issue contained the earliest print appearance of the delayed postfixed Not!
1893 Princeton Tiger (March 30) 103: "An Historical Parallel-- Not."