Obtained originally as a derogatory term used towards people of Asian culture (particularly Chinese), smog now refers to an insult used on people who are dickheads.
Used colloquially by eshays and normal people alike, the word 'Smog' shouldn't be thrown around lightly...
Also note: The insult is often best used with other rhyming words to show utter frustration at an individual. A bit like adding sugar to your coffee.
"My god, Jarred is being such a smog today"
"That guy was such a smoggin' floggin' dog am I right?"