That's a fun question! 806601 was from Sept 2009. I found 3 earlier cases of 'pivot' in the startup sense:

https://news.ycombinator.com/item?id=699611 (July 2009)

https://news.ycombinator.com/item?id=676514 (June 2009)

https://news.ycombinator.com/item?id=562739 (April 2009)

All 3 of those posts were by YC founders, so the term was obviously in circulation by then. The last of them includes a (broken) likn to this article: https://web.archive.org/web/20090703130211/https://redeye.fi....

Edit: that one was discussed here, but the comments didn't say the p-word:

Yogi Berra wisdom for startups - https://news.ycombinator.com/item?id=537331 - March 2009 (9 comments)