Presumably a very weird sex act. If it isn't, I'm sure that there all plenty of deviants prowling about there that will invent whatever the posh wellie is. I feel like it will have something to do with feet, seeing as a posh wellie is a nickname of sorts for Wellington Boots (I think) so it would make quite a bit of sense. It would also make sense for it to include rubbers, since Wellingtons are rubber boots. That's all I can think of, get to you glorious internet deviants.
Oh man, last night was a dream come true! We did the posh wellie for hours!
by Communistman July 22, 2014
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A fud that is really in need of a good clean. Like it's been out all night on a fisherman's foot as he's been trawling the seas.
Man, my new bird has a Grimsby welly. It nearly scalded my dee.
by AlBoy February 8, 2010
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Calf or Knee Lenght stilletoe heeled boots, usually black leather or a cheap white PVC among knackers. Irish slang for "Fuck Me Boots'
Oisin-"I see Sinead has the auld slut wellies on tonight"
Sinead-"Fuck you Oisin!"
by Saoirse Finnegan June 14, 2008
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Adidas superstars, worn by 99.9% of teenage girls who try to act hard
Are you wearing them slag wellies today?

Shut up! I love my superstars, call them that once again and I’ll bang you
by Fozzie77@ February 3, 2018
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A woman who has a large and damp Vagina not too disimilar to that of a Yeti's Welly
Yeah she was going down on me and i wanted to return the favour until i saw her Yeti's Welly
by paulobeef September 15, 2008
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Taken from 'Rogers Profanisaurus'

A poetic term describing a lady's bodily treasure.
'Spatula was powerless to resist. His eyes burnt into hers like mamellated chalcedony. His muscular arms enfolded her body as she felt herself being swept away on a noctilucent cloud of passion. "Am I the first?" whispered Dartanyan. "Sort of," she vouchsafed in return, her voice quivering with unrequited passion. "I've not let the other two have a shot on me melted welly yet."'
by sid Bollock November 13, 2004
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A clever wooden device for removing your wellyboots without getting your hands dirty.
Mum! Have you seen the welly wodger?
by The Glans Punis May 21, 2005
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