The Meaning of 'Jackson'

In a previous post I quoted a blog called Chart Rigger which reviewed Leon Jackson's performance with, "(He is ) rather a tall, dark geek like myself. Which means he's probably got a monster Jackson down his shorts." At the time I pondered if 'Jackson' was an American colloquialism for penis and whether this (alongside his inability to sing in tune) would affect Leon's chances of success in that country. I am grateful for the many readers who have written in saying the original poster should have used the word 'Johnson'.
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In fact a quick check of the Urban Dictionary confirms this with the definition of 'Jackson' being:
1) It's a twenty dollar bill, not a cock
2) Slang term for excellent as used on the Sit-com Nathan Barley
lots of Michael Jackson related meanings and...
3) a shit...as you in you bout to drop one, example useage, "boy, i gotta take a jackson.."