In fanfiction, canon characters acting out of character often take on strange and disturbing traits that stick out like a sore thumb. When this happens, the character appears so unlike him- or herself that a renaming is often in order. For instance, a trope of Lord of the Rings fanon is that Thranduil is evil and abusive to his son, Legolas. This warped characterization is then rechristened "Evil!Thranduil" by the PPC. Other common adjective!nouns are Evil!Denethor, Ho!Mione, and Stupid!Jack.
The adjective!noun construction originates in object-oriented programming, is pronounced [adjective]bang[noun], and means "the instantiation of [noun] that has the property of being [adjective]." It is used to emphasize that the canon version of the character does not have the property being described; only the version in the fanfic being referenced. An alcoholic Jack Sparrow, for example, would not be banged because canon Jack is an alcoholic. A version of Jack who did not drink would be Teetotaller!Jack.