"The Hat Fic" refers to an infamous fanfiction within the Dan and Phil fandom, known for being a shockfic with explicit content (smut). It gained notoriety for its bizarre and disturbing themes, including a character named Dan being tied up and forced to fill a hat with hamsters. Despite-or because of-its notoriety, it is often read out of curiosity or as a warning example within fanfiction communities. The story was originally posted around 2012-2013 and has been referenced multiple times by Dan and Phil themselves, as well as inspiring other shockfics since then

. In summary, "hat fic" is shorthand for this particular notorious fanfiction that is recognized for its shocking and explicit nature in the Dan and Phil fan community.