Anything which is repetitively posted, usually on imageboards like 4chan, to the point of annoyance or amusement, depending on one's point of view.
The thing is, people don't realize the work that goes into properly doing copypasta. They think copypasta is something that slackers can do, or faggots, or assholes. It's not true. Copypasta is a dying artform and if you don't see that, I don't know what's wrong with you.
First of all, you sacrifice spending real time on /b/. You can't participate as much as you'd like to because you're so busy doing copypasta that you can't. As a result, you miss a lot of really great threads. Still, it's a sacrifice, so you do it.
There's also the problem of "Flood detected". This message can really hurt your progress. You should try to get your copypasta into every active thread and if you have to sit there waiting before the flood period is over, you lose valuable time. This is also very difficult.
Also, picking which threads should get a copypasta first are sort of difficult. There are threads that don't stay on the first page for very long, so you may be missing some of the more prominent threads. Of course, you should try to hit them all, but for the desire effect, you need to get into bigger threads quickly.
Finally, there's the moral problem. One thing about copypasta is that sometimes it feels good, but sometimes it feels bad.
BTW, this wasn't a copy pasta, I just typed it out.
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