Another word for a vagina. The word derives from the acidic sting you feel when putting your tongue on a 9-volt battery. There is a somewhat acidic "sting" felt on your tongue when placed on a vagina because of the acidic environment (low pH).
For example: I went down on her 9 volt; Battery acid is leaking from her 9 volt;
A sexual menuever, where a man or woman takes a 9 volt battery, and places the two metal pronges on or in the womans vagina, giving her a mild shock, similar to the shock received when placing one's tounge on the battery.
"Dude, while my girlfriend was asleep, i thought i'd give her a little surprise by giving her the 9 volt."
When you're walking down the street and you see a chick with some rockin tits that literally make you turn your head and consider walking behind her just in case you get another glimpse. Those tits are 9 Volts.
She looked totally flat but when she took her shirt off all of a sudden she had a perfect pair of 9 volts!