The Brief: Online, the phrase "is cancer" can be used to hyperbolically emphasize how terrible something is.
To say that something “is cancer” usually implies that it’s very bad and harmful. The phrase often undermines the seriousness of the disease, by implying that something inconsequential is as devastating as cancer. Although it’s typically meant in jest, saying that something like a collaboration between TikTok and Fortnite “is cancer” can be insensitive to those who have been affected by it.
More formally, the term may be used outside of the context of internet meme discourse to make claims about something being “cancer to society.”