What does it mean, where did it come from?
Urby is an abbreviation of Urbanebula, a name I came up with when setting up my first email address.
It is a combination of the words Urban and Nebula, possible synonyms of City and Cloud respectively. A "city cloud" is basically smoke. So, in the grand scheme of things, Urby = Smoke.
(Some older members of TWHL may recall that many of my avatars featured clouds and skyscrapers.)