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1) Hanging upside-down whilst cracking one off;

2) Someone engaged in 1).

From the sink-holes of Surrey in England.
1) My back was killin', but I fancied a quick spaff, so I did the Jack-rack.

2) Dude, you're such a Jack-rack.
by Tuomas February 20, 2006
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My girlfriend caught me in mid stroke last night when I was in the jack rack, i could've died from embarrassment!
by ALL BIZ February 07, 2018
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