Definition of freak flag in English:

freak flag

noun

usually in phrase let one's freak flag fly
North American
informal
  • Used in reference to the open, proud, or defiant exhibition of traits regarded as unconventional.

    ‘you should embrace your weirdness and let your freak flag fly’

Origin

1960s: from freak + flag; earliest use found in the Jimi Hendrix song If 6 was 9 (1967).