A suffix used to denote an increasing number of subgenres of science fiction. It originated in cyberpunk, a subgenre of fairly pessimistic stories about gritty, bleak futures. Subsequently steampunk was coined by analogy to describe the emerging subgenre of pseudo-Victorian science fiction in the vein of Verne, almost certainly when two prominent cyberpunk writers (Bruce Sterling and William Gibson) wrote a book about the Analytical Engine called The Difference Engine, and from there, exploded into a profusion of more or less bullshit or popular grotesqueries such as dieselpunk, atomicpunk &c.
For some reason, it's fairly common to see assertions that the suffix is meaningful in each instance and a deliberate reference to the short-lived punk subculture (in the worst cases, asserting a requirement of being critical of capitalism, obviously because the writer's own ideological inclinations have consumed his head whole), but in fact this is completely and demonstrably incorrect. In every case but the originator, it's a mere calque off cyberpunk and used to denote a stylistic similarity — if even that. For example, in steampunk, the tag was used to imply »cyberpunk technology but it's all made out of gears and boilers and shit!«, so that people would have clockwork prosthetic arms and (importantly) Babbage computers, but steampunk only rarely contains anything like a critique of capitalism or class relations and in fact is mostly about very rich Vernian inventors flying around in balloons, inventing crazy Vernian shit, having adventures and so on, and the stories are normally highly optimistic. Dieselpunk, in turn, was calqued off steampunk and doesn't really contain any connection to either punk per se or cyberpunk; most of it just involves blasting around on gigantic smoke-belching diesel-driven vehicles of various kinds wearing overalls and waving a giant wrench around. Even cyberpunk itself was an early victim, novels not written by Gibson being very prone to just embracing all the sweet technology and gritty æsthetics to make the protagonist look super awesome to a twelve-year-old, and the setting just cool as hell to live in. (Nerds have notably never been particularly receptive to critiques of VR and metaverses as hazardous escapism, but have preferred to use these scenarios to dream of the totally rad escapism they hope will be available in the future.)
Most of these subgenres are also characterized by limited lifespan. If cyberpunk (which was certainly the biggest and most successful among them, in its time) isn't a deserted genre it's certainly very close; steampunk took two or three years at most from its explosive popularization in the early Web 2.0 era to becoming a laughing stock even among nerds, with epithets like »cogfop« hurled at its awkward fans. Nobody has ever liked dieselpunk. One day perhaps we will see the rise and almost immediate collapse of punkpunk, a movement to restore the punk ethos to the -punk subgenres.