WHEN did grunge become grunge? How did a five-letter word meaning dirt, filth, trash become synonymous with a musical genre, a fashion statement, a pop phenomenon?

From subculture to mass culture, the trend time line gets shorter and faster all the time. It was just over a year ago that MTV began barraging its viewers with the sounds of Seattle "grunge rock," featuring the angst anthems and grinding guitars of bands like Nirvana and Pearl Jam. By last summer, the glossy magazines began tracking grunge looks, the threadbare flannel shirts, knobby wool sweaters and cracked leatherette coats of the Pacific Northwest's thrift-shop esthetic. Hollywood weighed in, too, with a grunge-scene movie, "Singles." Then two weeks ago -- all in the blink of a flashbulb -- the fashion designer Marc Jacobs, who has never even been to Seattle, was hailed as "the guru of grunge."

All this has happened before, with the mass-marketing of disco, punk and hip-hop. Now, with the grunging of America, it's happening again. Pop will eat itself, the axiom goes. Here's how it feeds:

In 1988, a fledgling Seattle record label called Sub Pop released a three-boxed set called "Sub Pop 200." It was a compilation of bands like Nirvana, Soundgarden and Mudhoney, and it came complete with a 20-page booklet packed with pictures by Charles Peterson, the photographer credited with creating grunge's hair-sweat-and-guitars look. Sub Pop also sent a catalogue to the nation's alternative-rock intelligentsia describing its bands' punk-metal guitar noise as "grunge," the first documented use of the now-ubiquitous term. "It could have been sludge, grime, crud, any word like that," said Jonathan Poneman, a Sub Pop founder.

Grunge stuck, maybe because it so vividly evoked both the black-noise sound and the smelly-caveman look. Ratty rec-room chic has been hibernating since the 70's, emerging from the basement every so often in movies like "River's Edge," "Bill and Ted's Excellent Adventure" and "Wayne's World."