the fuck

Short definition from Wiktionary

Phrase

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