First Attested Usage?

William Gibson and Bruce Sterling, The Difference Engine, page 23:

Mick straightened, frowning at her. “Now that’s exactly what I mean. You’re running with the flash mob, now, but thinking like a trollop! …”

Emphasis added.

Nice find.

Larry Niven was using the term “flash crowd” looong before them…. short story in the ’70s or ’80s.

I don’t think that really counts because it’s using the words in a very different sense. “Flash,” in that world, was a slang term roughly meaning “cool.”