I only heard the above phrase recently but I am guessing it has been around as long as wargaming, it may be our very own version of 'anything goes'.
If you don't know what it is it is being able to paint your miniatures any way you like as you bought them and stuff the rest, it probably also applies to rule sets and many other facets of our hobby but it is primarily the painting that interests me.
I have noticed on the Bolt Action Facebook page especially, and just the other day a guy asked if he should have his Kriegsmarine NCO in a white cap or a dark blue cap, I am unsure of the point of the exercise because he had already painted it white. Right away it was pointed out he was wrong because only Captains and above were allowed white, to which he countered with several excuses as to why the guy would be wearing white, he might even be a Captain fallen on hard times etc. then why did he ask when he knew it was wrong. The upshot of course was several replies along the lines of 'they are your troops paint them how you like', and anyone who disagreed was an historical fascist.
I have seen this kind of thing on many an occasion, for instance someone will showcase a German tank, like a Tiger painted in real tiger stripes and then they ask the question, what do you guys think? Well I think that is stupid is the first thing that comes to mind, next is why on earth would you do that with all the evidence to the contrary. Another wanted to do his entire German army in black and then tried to persuade the rest of us that Hitlers bodyguard, from the propaganda films before the war, actually went to war like that, or could have, prove they didn't. And of course he was told go ahead!
There are whole swathes of history where we don't really have much of a clue, no one can be sure if Romans wore red or had different shields for different cohorts, but we do know they were not baby pink, it is fairly easy with the internet to make an educated guess.
I don't get Imagi-Nations but I have no problem with someone building up their army the way they want and having fun mustering their own regiments, I also don't care that my opponents troops might look as though they were painted with a two inch brush, but don't put the Black Watch in yellow kilts or Union troops in turquoise, it is just wrong.
I have never in all my years of wargaming bumped in to one of these guys, would I refuse to play against someone who turns up with a tiger Tiger, just maybe.