the fuck

Perhaps modelled after the hell, short for in the hell as in "What in the hell, ...".

1Used with an interrogative pronoun as an intensifier to express anger, frustration, incredulity, or other strong emotion.
2Used after verbs forming a part of a phrasal verb as an intensifier to imply aggressive emphasis.
3by ellipsisUsed as a shortened form of the common interrogative phrases.
4Used to emphatically express that something isn't true.
5African-American VernacularUsed to emphasize an adjective; fucking.