To engage in anal sex without permission while having regular intercourse.
This phrase originates from the bases where fourth base, or home base, describes sex. If you move past this by having anal sex, you've reached fifth. To do it without permission, you stole fifth.
"I was behind this girl doin' it doggie style last night. I'd never put it in an ass though, and I was kinda curious so I had to try stealing fifth."
"How did that go?"
"She let me stay for extra innings"