Initially developed out of spite due to the likely verbal and physical attacks from 'chavs' or inebriated followers of mainstream activity, the term now seems to be applied with a sense of pride, to the extent of labelling and in some cases returning the verbal and physical abuse to the supposed 'trendy'.
The obvious irony in this statement is that, as is aforementioned, a select niche of society (namely, the alternative society)use this phrase almost exclusively (at least in this context) and yet they use it to somehow proclaim their individuality.
The truth, it would seem, is that it is an excuse. A way of excusing the fact that the populous don't like the way you look and the way you dress. They have their own reasons i'm sure, some well-founded, some slightly bigotted but still individual reasons that aren't incited by the media (no 'freak-bashing' music videos on MTV for example, or articles about how wrong dyeing your hair blue in the National Press). Due to this seemingly unanimous disapproval, this term is used to imply that the offending party doesn't like them because they are a trendy, and the accusing party is so out-there and hip and open-minded that the spoon-fed, publicised masses simply couldn't understand.
An element of denial is involed as well, the subculture referred to earlier has been created out of a need ot be accepted. The majority of the users of this term have not been accepted generally by their peers and seek others to share this feeling. The numbers rise and rise and before you know it, we have a group large enough to huddle together in there issues with other people and shield themselves from it with boundaries, borders, narrow-mindedness and phrases such as this.
They believe that they are doing what they do becuase they don't like what has been 'dealt' to them and are un-impressed with their peers, their government and their country in general, but that is a fantasy (too much Bad Religion can go to your head ;)). It is largely due to the fact that their extreme sense of dress WILL be accepted by a much smaller section of society, and therefore they can join the huddle. Rather like penguins.
In short, if you call someone a trendy and mean half of the emotive and spiteful definitions listed here, chances are you deserve to be treated with the same narrow-mindedness that you treat the rest of the world with.
(this can all, of course, be denoted from what the guy wears)
Trendy girls, well thats just an entirely different definition, personally i avoid most of them.
Tom: Its erm trendy??:S
Nick: You don't have a clue.