A sexual manouever in which a man, while fingering a woman outside of her field of vision, inserts his penis without announcing he is doing so. The act is generally employed as a means to commence intercourse with a hesitant (but willing) partner.
The term "eleventh finger" can also simply be slang for penis.
Chris: Well, she was kinda riding the fence about having sex with me. Since she never said no outright, I gave her the eleventh finger, and that was that.
Kevin: nicely done, sir.