Real Definition: A botnet is a system of machines that were hacked to perform a malicious purpose, such as ddos, child porn, or spamming. How these systems get connected to the leader of the botnet is through malware filled programs, or by finding your IP by means of cookies or programming on a website. Hola! proxy is a notable example of a botnet.
How It’s Used As A Meme: 4chan and such websites have turned this malicious term into a meme, such as: saying anything that collects data (such as Google Chrome) is a botnet, and insulting anybody that uses 3rd party by saying: “>Using a botnet”.
People that support 3rd party will usually counter back with “>le botnet meme” or such trolls, because Google Chrome and such 3rd Party Programs are not really botnets.
The origin of how it turned into a meme is undefined.
Anontalk: Anontalk botnet is a famous botnet that got a lot of random victims banned from 4chan. The website that got shut down hosted child pornography, or such related acts of child pornography. The Anontalk botnet probably downloaded or helped download child pornography, or it had some other undefined malicious purpose that it served.