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2.
activities in which on man partakes in sexual relations with themselves
Charles was banished after being caught participating in jacktivities at the playground
by The T.I.P. Drill October 25, 2009
 
1.
The amalgamation of the word activity, and the British Army slang word "Jack" meaning selfish or lazy.

A jacktivity is something a multitude of people will do in order to evade hard work.
Person 1: "While we're in here, the smokers go outside every 5 minutes and evade the hard work."

Person 2: "That's such a jacktivity."
by Nicko678 December 13, 2009
 
3.
1. an assigned task that is nothing but a worthless, waste of time jack-off activity.
1. There was nothing worthwhile to do at work today so the boss assigned me a jacktivity to keep me busy.
by ETQuall March 21, 2012