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?!"
Short-hand for the word floccinaucinihilipilification, meaning the estimation of something as valueless (encountered mainly as an example of one of the longest words in the English language).