when you've had too much to drink and have a girl back home and cant get it up to perform the deed

When too much alcohol has been consumed into a males body making it impossible to achieve an erection even in circumstances where it would be almost impossible not to.

the ultimate form of birth control

Getting so drunk that you can't get it up.

Originally this condition was used to describe the extreme shrinkage caused by drinking alcohol, turning even the largest dicks into mere helmets in the bush.

More recent usage of the word has devolved to a more general description of being too drunk to get a hard-on

1 - A gentleman has too much to drink and has trouble getting an erection at the lady's request and instead just falls asleep.

2 - A gentleman drinks some, but not too much. The erection arrives to the lady's delight and lasts much longer than normal. If gentleman doesn't cum during the exchange its ok. Blue Balls don't exist for this situation.