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Based on the sound /eɪn/, common to the verb, adjective and noun that compose it, the slang phrase to drain the main vein means: to urinate.
In this phrase, the expression main vein probably refers to the penis. The following, for example, is from the column Eating Owt, by Mike Amos, published in The Northern Echo (Teesside Edition) (Darlington, County Durham, England) of Tuesday 1st September 1998 [page 9, column 2]—the New Inn is a pub in the city of Durham:
The New Inn, in truth, now bears passing resemblance to a fairground with a liquor licence.
There is constant, insidious music, assorted and egregious screens, multi-coloured walls, electronic machines the size of iron lungs (though of considerably less benefit to humanity), writing not just on the wall but on the ceiling, too. Not so much a pub as a playschool for teeny boppers.
The gents has been re-named the main vein drain, the shabby tables are carved or stained with endless, mindless, graffiti.
These are, in chronological order, the earliest occurrences of the phrase to drain the main vein that I have found:
1-: From The Green Tortoise: Alternative transportation company guarantees its trips to be ‘nothing like your last bus ride’, by Stephen Magagnini, published in The Sacramento Bee (Sacramento, California, USA) of Sunday 23rd April 1989 [Travel section: page E1, column 2]:
San Francisco—From the outside, the Green Tortoise could be any broken-down transit bus with two million miles on the odometer (except there is no odometer).
Inside, the Tortoise is a rolling commune, a time machine that transports passengers back to the ’60s. For $49 you get a Woodstock Nation on wheels, a 25-hour trek from San Francisco to Seattle via the Summer of Love.
[…]
[…] Before we leave the terminal, our lead driver, “Captain Curt,” a.k.a. “Naked Man,” goes over the ground rules: […]
[…]
“We take breaks every 1½ to two hours for the smokers and the pee-ers,” Curt says reassuringly. “For the pee-ers who can’t wait, we have The Funnel, a non-sexist device—I’ve seen women negotiate it better than some guys. […] WE BE ROLLIN’!”
At 11:11 p.m., after picking up a few more passengers in Berkeley, Curt announces, “It’s time to drain the main vein and make the bladder gladder!” and we take a 10-minute rest stop in a Fairfield shopping center.
2-: From Road House (released on Friday 19th May 1989), a U.S. action film starring the U.S. actor, singer, songwriter and dancer Patrick Swayze (1952-2009) as James Dalton, and the Canadian guitarist, singer and songwriter Jeff Healey (1966-2008) as Phil Cody, the house band’s frontman.—In the following, Cody announces a short break [at 11’45’’]:
We’ll be back in 10. We got to drain the main vein, man.
3-: From the column Dog-eared Tales, by Bill Studebaker, published in The Times-News (Twin Falls, Idaho, USA) of Thursday 1st May 1997 [page D1, column 1]:
A few days ago, a woman called to say she had a puppy that was a submissive piddler. […]
[…]
[…] She was tired of mopping up dog water. She’d run out of paper towels and she wasn’t buying any more.
She said she’d swatted the pup a few days before, then rubbed its nose in the puddle and threw it outside. Even though it was cold, she made the pup stay outside for an hour and a half.
It didn’t help. In fact, the problem had escalated to the point that whenever she looked at the pup, it would squat and drain the main vein.
4-: From a review of Dickie Roberts: Former Child Star (released on Friday 5th September 2003), a U.S. comedy film starring the U.S. actor David Spade (born 1964) as Dickie Roberts—review by Marke Andrews, published in The Vancouver Sun (Vancouver, British Columbia, Canada) of Friday 5th September 2003 [page D3, column 2]:
Dickie pays a family $20,000 so he can live with them for a month to see what “normal” is all about.
Dickie’s arrival pleases only the absentee man of the house, wayward husband and father George (Craig Bierko). For the others, good-looking mom Grace (Mary McCormack), pubescent son Sam (Scott Terra) and pre-pubescent daughter Sally (Jenna Boyd), Dickie’s crudity and worldliness don’t exactly fit their family values. Announcing his move from the dinner table to the bathroom, Dickie tells them, “I’m gonna drain the main vein.” Settling into his waterbed in the children’s bedroom, he lusts aloud for Sally and Sam’s mother.
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