Definition
By extension, any last chance or final opportunity.
Origins
Bottom, from the home team's location below visitors on the scoreboard; ninth from the number of innings in a baseball game.
How People Actually Use It
Noun - an event that takes place at a bar when you have struck out all evening and need to score. At this time you start looking for any member of the opposite sex to mack on and try to hook up with
"Last night I was at the bar and it was the bottom of the ninth so I hooked up with some coyote just to get some."
Source: Urban Dictionary