(vulgar, slang) Used after interrogative pronouns as an intensifier to express anger, frustration, incredulity, or other strong emotion.
Synonyms: see Thesaurus:the dickens
(vulgar, slang) Used after verbs forming a part of a phrasal verb as an intensifier to imply aggressive emphasis.
Shut the fuck up.
(vulgar, slang, by ellipsis) Used as a shortened form of the common interrogative phrases.
The fuck was that? ― What the fuck was that?
The fuck you think you are?! ― Who the fuck do you think you are?
The fuck? ― (said incredulously; also as what the fuck?)
(vulgar, slang) Used to emphatically express that something isn't true.
Source: en.wiktionary.org