Used to describe being targeted by a particularly clever insult or witticism that uses a portmanteau, or combination of two or more words, as a punchline. It can also be used sarcastically, for example after an especially weak pun.
The word is a portmanteau itself, of "portmanteau" and "owned." As a result, timely usage of the word can count as an example of itself.
"Dude, there's a poltergeist in my chicken coop!"
"Don't you mean a...poultrygiest?"
"Oh, shit, I've been portmanteauwned!"