context: "[Author Warren] Ellis's [super]heroes would happily cut off your head and beat you to death with it if that's what it took to keep you from being a dictator or a 'bastard.' These hombres meant business … These bolshy new superheroes spoke for all of us in the counterculture … It felt like we'd won."
definition from the free dictionary: obstreperous - obstinate, stubborn, unregenerate - tenaciously unwilling or marked by tenacious unwillingness to yield; from Bolshevik, a revolutionary or radical.
The word is a Britishism. Grant Morrison is a Brit, so that makes sense.
I'm not sure I can characterize what Morrison means when he says it felt like "we'd won." Down the page from the above quote Morrison says, "The twenty-first century would surely see the triumph of our sci-fi ideals along with the death of grim, old, outmoded power structures, and [Warren Ellis's] The Authority spoke for that dream." Tres bolshy?
source: Supergods: what masked vigilantes, miraculous mutants, and a sun god from Smallville can teach us about being human by Grant Morrison