Jacksie is Cockney Rhyming Slang.
It means backside, bottom, arse...
The word comes from : Jacksie = Jack and Danny = Fanny.
It's not a synonym for fanny though.
You can shove your Sony PSP right up your jacksie
by Argun August 04, 2006
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A Jacksie is a person who can manoeuvre their penis down between their legs, curling it around and inserting it into their own anus. Essentially it's the act of fucking yourself
"oh man, did you see Phil put his dick in his own ass the other day?!"

"Yeh, he's such a dirty Jacksie"
by Julesyyyyy June 04, 2018
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