The oldest instance of "based off of" that a Google Books search finds is from "Negotiation...... It can Work for Principals!!" in Minnesota Elementary School Principal, volumes 15–16 (1972) [combined snippets]:
- Establishment of a new salary schedule with a ratio based off of the average salary of the superintendent and three (3) assistant superintendents. (Previous salary based on ratio off of the teacher's schedule.)
Interestingly, this instance of the phrase "a ratio based off of the average salary" is followed by the almost equally odd phrase "salary based on ratio off of the teacher's schedule."
The next instance occurs 12 years later, in Long-term Farm Policy to Succeed the Agriculture and Food Act of the Agriculture and Food Act of 1981: Hearings Before the Committee on Agriculture (1984) [snippet view]:
What you have is a program where acreage base is based off of planted acres, but yield is based off of harvested acres. You've got two common denominators—or not common denominators.
And the next match is from Doug Kinnison, Effects of Trace Gases on Global Atmospheric Chemical and Physical Processes (1989) [text not visible in snippet window]:
The reaction set is based off of JPl-87. The scenario for both cases A and B, inject NOx globally at 19.5 km, with a magnitude of 1.8 Mt yr".
Google Books matches become more frequent in the 1990s, starting with this one from Alyeska Pipeline Service Company Covert Operation: Draft Report of the Committee on Interior and Insular Affairs of the U.S. House of Representatives (1992) [combined snippets]:
These records have additionally been ordered. Lund also obtained a verbal report of the telephone records belonging to Robert Scott, based off of the preliminary analysis conducted by Alyeska investigators.
April 23, 1990: returned to their hotel suite to await any possible contact as a result of placing flyers. No calls or messages were received. During this time Investigators Lund and Crep created a chronology toll call analysis based off of preliminary reports submitted by Lund on Hamel's telephone traffic.
Hamel confided that he met and learned about Miller due to some of his interest in terminal operations and environmental issues. He developed a relationship with him whereupon he supplies information to Miller based off of the stolen documents from Alyeska.
Google Books also returns one match from 1994, two from 1995, one from 1996, and one from 1997, from such sources as the U.S. Department of Energy, the Society of Actuaries, the U.S. Senate Committee on Finance, Dairy Pipeline magazine, Standard & Poor's Creditweek.
The earliest sources of the expression "based off of" strongly suggest that the phrase originated not on children's TV but in the wilds of Minnesota, Alaska, and the U.S. Congress. The logic of the expression may be that the thing so described is not "based on" some other thing so much as it is "broken or branched off [or off of]" it; but somehow "based" persisted with "off of" in place of "on."
Whatever the rationale underlying it, the phrase seems not to owe its existence to suburban teens, big-city hipsters, or Hollywood scriptwriters. Its visible roots are rural, scientific, and legislative.