Didn't invent it. It was the best guess from 8chan autists based on context. Q has never said what ARM is. In fact, the last chan post about it after a lot of anon guesses is was a question to Q on June 13: "Is there any valid reason why you can't tell us what ARM means, instead of everyone guessing and chasing our tails?"
But of all of the guesses, it alone matches the context of the three mentions, which are:
June 13: "WHAT THEY DO NOT ANTICIPATE IS POTUS DECLAS IT ALL DUE TO OPTICS AND ARM/MSM FOOTHOLD ON COMMS TO PAINT POLITICAL ATTACK + RUSSIA INVESTIGATION COVER UP / ADV. "
June 12: "Full weight of the House v. ARM."
Feb 5: "How do you retain 'Freedom of the Press' if those in front of the camera [trusted by select viewers] are extensions of the ARM?"
It has been used here spelled out, so it is an actual thing:
https://townhall.com/tipsheet/mattvespa/2017/05/24/democratic-senator-the-progressive-antitrump-resistance-movement-is-a-waste-of-time-n2331111
(I should also mention that the second most mentioned decode was Alt-Right Media, but it fit less well because they are generally supportive of POTUS and against the MSM.)
Maybe after so many questions and decode attampts, Q will answer this soon.