From the second edition (1989):
(ˈdʒæksɪ) Also jacksie, jacksy-pardo, -pardy, jaxey, jaxie. [f. Jack n.1 + -sy.]

The posterior, backside, arse.

1896 Farmer & Henley Slang IV. 33/1 Jacksy-pardy, the posteriors. 1943 Hunt & Pringle Service Slang 40 Jacksie, service slang for ‘rear’, ‘tail’, or ‘bottom’. 1959 K. Waterhouse Billy Liar v. 78 Why don't you tell the boring little man to stick the job up his jacksy? 1963 Sunday Times 15 Sept. 29/8 Tonbridge boys many years ago said ‘a root on the Jaxie (or Jacksie)’ for ‘a kick in the pants’. 1966 B. Naughton Alfie xxxvi. 206 She's sitting there on her jacksie, reading one of those colour things out of a newspaper. 1970 A. Draper Swansong for Rare Bird i. 9 The amount of love in our house you could stick up a dog's jacksie and he wouldn't even yelp.