the fuck

(vulgar, slang) Used after interrogative pronouns as an intensifier to express anger, frustration, incredulity, or other strong emotion.

(vulgar, slang) Used after verbs forming a part of a phrasal verb as an intensifier to imply aggressive emphasis.

(vulgar, slang, by ellipsis) Used as a shortened form of the common interrogative phrases.

(vulgar, slang) Used to emphatically express that something isn't true.