I guess it's a short for クソリプライ which literally means shit reply.
But what is "shit reply"? Is it similar to "shitpost"? I don't really understand its meaning along with where and when you would use this phrase.
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It's pretty much self explanatory: a shitty reply. I'm not aware of what shitpost means, but looking at the definitions online, I would say they are quite similar.
The difference is that this only relates to replies. This term is mostly used in regards to replies on Twitter that are irritating, that are boring, that amount to nothing. Some people may or may not consider certain replies as クソリプ.
For example, if I tweeted:
I went to Starbucks today and ordered a large ice coffee unsweetened, but they still sweetened it.
Examples of what I might consider a クソリプ would be:
1) A reply with just a face like (^^; in it.
2) Some stranger butting in out of nowhere and telling me that "it's not 'large', it's 'venti'."
3) Someone blaming me, taking the side of the worker (despite me not actually saying anything bad about the workers)
As for when to use it? I'm not sure there's ever a time I'd use it. It's net slang, which means the only time ever acceptable would be online, but if were to ever used it'd be when I tell an online friend about the replies and say 「見てくれ、このクソリプ」(Look at these shitty replies).