From what I know, a trigger is something that makes you remember a traumatic past experience..

Most people, usually those without MI, twist it around and overuse it, to refer to some messagges that they don't like (not even upset them, just don't like), so nobody takes it seriously anymore. (That's why you'll find many people that makes fun of it)

I think that's terrible, especially because those who are actually triggered, like the folks here at PC, aren't taken seriously when they say they're triggered.. that makes me mad.