When a person has intercourse with their 2nd cousin.
Bob was 'whistlin' dixie' with Carol last tuesday.
To be speaking in jocund terms about something more serious. To speak of something in a manner less serious than it should be, while still maintaining the adamancy of your beliefs.
Mike: Those FAMU boys actually think it's their right to have homecoming in this town again this year.
Josh: Boy, you ain't whistlin' Dixie.