|Synonyms:||bang, exclamation point, adjective!noun|
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An exclamation mark (sometimes called a "bang") between two words denotes a trait!character relationship between them, especially between a character and a trait of that character. For example, CAPSLOCK!Harry refers to Harry Potter shouting in capslock during much of The Order of the Phoenix, while Femme!Blair would refer to a characterization of Blair behaving in fanfiction in a way some might consider stereotypically effeminate. The idea of Dark!Harry has inspired hundreds of fanfics.
The bang relationship can also be used in the noun!fic form, referring to things like wing!fic, crack!fic, rape!fic and so on. However, in this case, it's usually also acceptable to omit the exclamation mark and make it one word, which does not happen for trait!character expressions.
X-Files Origin Story
The adj!noun format is purported to have started in X-Files fandom. Good examples for this are Wombat's Spotter's Guide to the Common Krycek and the accompanying Spotter's Guide to the Common Mulder where several Krycek sub-species such as Bad to the Bone!Krycek, Bad but Lovin'!Krycek, Hot'n'dirty!Krycek or Misunderstood!Krycek and Mulder sub-subspecies such as Angst!Mulder, Basketcase!Mulder, HappySlut!Mulder, Sensitive!Mulder and WellAdjusted!Mulder are described. The first usage was Action!Mulder, referring to canon scenes where Mulder suddenly went all actiony (instead of talky), followed by Saint!Scully. Eventually they lost the initial capitals and the canon connections; by the time it hit other fandoms, it was being used to describe fannish things.
Another X-Files mention, this one from the FAQ for OBSSE in 1997: "'Where did "!Scully" come from?' This is a sort of shorthand for describing our favorite characters, depending on what they were doing (such as Action!Scully or Kickass!Scully). It's sort of a designation for hypothetical action figures. For more information on some of the Order's favorite hypothetical action figures, see the October 1997 OBSSE Newsletter Poll." 
Email Origin Storyraincityruckus posted on Tumblr, Archived version
- “Bang paths” (! is called a "bang" when not used for emphasis) were the first addressing scheme for email, before modern automatic routing was set up. If you wanted to write a mail to the Steve here in Engineering, you just wrote “Steve” in the to: field and the computer sent it to the local account named Steve. But if it was Steve over in the physics department you wrote it to phys!Steve; the computer sent it to the “phys” computer, which sent it in turn to the Steve account. To get Steve in the Art department over at NYU, you wrote NYU!art!Steve- your computer sends it to the NYU gateway computer sends it to the “art” computer sends it to the Steve account. Etc. ("Bang"s were just chosen because they were on the keyboard, not too visually noisy, and not used for a huge lot already).
- It became pretty standard jargon, as I understand, to disambiguate when writing to other humans. First phys!Steve vs the Steve right next to you, just like you were taking to the machine, then getting looser (as jargon does) to reference, say, bearded!Steve vs bald!Steve.
- So I’m guessing alternate character version tags probably came from that.