It's true: Everywhere you go in the world, this term is used as a standard form of address in speaking to straight white males. Mostly, it's propagated by the liberal media. When I can't turn on my television, set foot on a college campus, or walk into a restaurant without being called an "imperialist pig" by half the people I see, there's a real problem there.
The origin of this term lies in the great age of European "colonialism" in the 18th and 19th centuries. During those years, the benefits of European civilization were brought to a majority of the world's population. In countries like India and China, thousands of years of brutality and barbarism were replaced by enlightened rule and free markets. Naturally, the nabobs and pandjandrums of the Old Order in these backward nations were horrified, and they fought back. The so-called "Opium War" in China is a case in point. Leftist revisionist historians have painted a wildly inaccurate picture, hijacking history so far as to invent a fictional name ("Opium War") for what was, in fact, a Free Market War. The Mandarins were fighting to regain their stranglehold on the economy, while the English merely strove to preserve the market liberalism they'd so recently introduced. Opium had nothing to do with it; the "histories" you've read are a mass of spin control and ideologically motivated gibberish.
So, "imperialism" is really the practice of freeing third world peoples from the horror and slavery of their barbaric past. "Imperialism" is our greatest pride, our greatest achievement, our generous gift to the myriad peoples of the world. The Leftists hate that. Oh, boy do they hate that. So they coined this term, "Imperialist Pig", in an attempt to hijack the language and distort reality.
The next time you walk into Denny's and the welfare mother waitress addresses you as "Imperialist Pig", smile. She doesn't know it, but she just paid a great compliment to your committment as a white male to improving life for all the human race.