n. A person who believes that the world's oil reserves will soon peak and that subsequent oil shortages will devastate civilisation.
If the day comes that oil grows so scarce that Austinites can't afford fruit hauled in from California and brownouts roll across Texas, Lester Germanio will live high, wide and cool in his West Lake Hills villa.

Germanio and other Austinites who have banded together to trade information and survival tips are preparing themselves for what they see as inevitable deprivations as oil production declines past its peak. Some call them "Peakniks." …

As the anticipated peak oil crisis unfolds, Peakniks foresee a period in which the U.S. would devolve into the stone age. The economy would tumble; cars, even hybrids, would be useless; day-to-day goods would be hard to come by.
—Asher Price, “Should oil wells start running dry, Austin's 'Peakniks' will be prepared,” Austin American-Statesman, February 10, 2008
Paul the Tree comes by to give us a talk on how to make biodiesel. He takes waste vegetable oil from local chip shops and mixes it with oil. He is a peaknik — someone concerned about peak oil and its consequences. Like Agric, he believes we are at peak oil already.
—Dylan Evans, “Are visitors welcome or is this Noah's ark?,” The Times (London), May 25, 2007
2006 (earliest)
M. King Hubbert, who died in 1989, acquired a global following among earth scientists and conservationists — nicknamed peakniks by skeptics — for making a correct prophecy about oil in the United States. The question is whether the theory also applies to world oil supplies.

In a 1956 paper, Hubbert predicted American production would hit maximum volume and go into decline in 1970. He came within a year of hitting the bull's-eye. In hindsight, the turning point was 1971.
—Gordon Jaremko, “Oilsands put Alberta in the driver's seat,” The Edmonton Journal, February 22, 2006
Back in the early 90s, when the oddball TV show Twin Peaks inexplicably gripped the world's attention, devotees of the show (and they were legion) were sometimes called Peakniks. Here's the earliest citation I could find for this sense of the word:

While San Diego Chargers boosters can revel in the news that KGTV (Ch. 10) is airing Saturday night's preseason game against the Los Angeles Rams in Anaheim, devotees of ABC's "Twin Peaks" have been furious, thinking that the exhibition was going to mean they would miss a pivotal episode of their favorite program. Rest easily, Peakniks, you're going to get to see your show after all.
—Joe Stein, “Chargers, 'Twin Peaks' get their signals straight,” The San Diego Union-Tribune, August 16, 1990