When you read up on this question you run into a lot of hokum about how Toyota high-performance projects going back fifty years had the number "86" somewhere in their nomenclature.
But here's a much more compelling explanation.
I think Toyota's branding department went into the history books and dug up the story of an authentic street-racing culture that was thriving fifty years ago on Highway 86.
Right in front of my house.
Makes perfect sense, doesn't it?