A WebMD article by Hallie Levine (re)defining “brain rot”, a term coined in 1854 by Henry David Thoreau. It also covers what it was, what it is now, examples of how it manifests and its effects and how to prevent it. The change of definitions n interests me: “[Thoreau] defined “brain rot” as liking simpler ideas instead of complex ones. … Brain rot now describes dulling your brain function.” Which suggests that the definition has slightly changed with the evolution of technology. 

Source: webmd.com