What are slang terms that people on Mi Yodeya use in comments or chat that are unique to this community?
Please post one term with definition, etymology, earliest known usage, etc., per answer.
Please only include terms that can reasonably be considered to genuinely be part of the communal lingo. For example, one known usage of a term or your invention of what you think would be a neat term are not good enough.
The goal of this post is to create a searchable reference that people can discover if they encounter one of these terms in the wild and have no idea what it means.