Really, really old. Beta is the test phase before a product release to the general public. "Older than dirt" refers to someone or something really ancient - but dirt-beta is even older.
"Man, those clothes are so out of style they're dirt-beta."
"Dude, your grandpa is so old, he did QA on the dirt-beta."
"Dude, your grandpa is so old, he did QA on the dirt-beta."
by Captain Frank April 25, 2008