When and how did the phrase "off the meter" become established as an idiom?
Urban Dictionary defines "off the meter" as the condition of being "very good, awesome, great". I have heard and said it on impulse to mean the great extent.
I can think of three possible senses of "off the meter" that might account for the current popular phrase: (1) out of bounds, beyond the dial readings of a light, sound, or other type of gauge, as when the indicator arrow on a meter seems stuck at its extreme high end; (2) unregulated, as when a cab driver turns of the car's fare meter and drives for a special price or for free; (3) syncopated, so that the emphasis of the rhythm falls elsewhere than on the expected beat.
The meaning cited as possible source #1 appears in Frank Herbert, The Dragon in the Sea (1956) [compiled from snippets]:
The meaning cited as possible source #2 appears in Jerry Oster, Sweet Justice (1985), reissued a year later as Rough Justice [compiled from snippets]:
The meaning cited as possible source #3 appears in Michael Tenzer, Gamelan Gong Kebyar: The Art of Twentieth-Century Balinese Music (2000):
One of the first metaphorical use of "off the meter" that a Google Books search finds is from Sean Henry, "Comic Threat," in Mother Jones magazine (November/December 1994):
I think that this instance of "off the meter" probably derives from possible source #1 above, and I suspect that current usage derives from the same source.
Consistent with terpy's answer, the first instance of "off the meter" as a synonym for "very good, awesome, great" to appear in a Google Books search is this one, from a caption in "The Vibe Spot" in Vibe magazine (August 1998):
Off the meter, off the chain and other expressions like that grew out of Hip Hop culture.