I am 51 yrs old, stood in line to watch the original Star Wars movie, now called Episode IV: A New Hope. Although, it was the first instance of a Jedi Mind Trick by Obi-Wan Kenobi (Sir Alec Guiness) when he waved his hands in the air to say to the Stormtroopers, "These are not the droids you are looking for.", i believe the term JMT first mentioned in Episode I: Attack of the Clones when Qui-Gon (Liam Neeson) tried it on Watto and Watto said words to the effect of: Why are you waving your hand in the air. I'm of this race, your JMT don't work on me.
The usage in culture is based off of those 2 movies. It's as Shuki and UpSideDownCarl said ... you 'psyched out' your opponent.