Showing covert hostility, intending to cause annoyance or to humiliate, while maintaining a transparent (to the recipient) veneer of politeness.
“I Love Lisa” opens with one of my favorite underappreciated running jokes from The Simpsons: the passive-aggressive, quietly contentious relationship of radio jocks Bill and Marty, whose mindless happy talk regularly gives way to charged exchanges that betray the simmering resentment and disappointment perpetually lingering just under the surface of their relationship.
Curt texts, unanswered emails, Facebook statuses declaring “someone” to be an idiot, the notorious subtweet; if you can communicate with it, people can be passive aggressive with it. No doubt there were frosty telegrams in centuries gone by.