Completely unrestricted and able to do whatever one wants, without constraints or obligations.
This simile has been used since at least the 16th century, drawing from the obvious observation that birds can fly wherever they choose. The phrase gained particular popularity during the Romantic period when freedom and nature were idealized themes in literature.
Ironically, many birds are actually quite restricted by territorial boundaries, migration patterns, and seasonal behaviors - they're far less 'free' than the phrase suggests. The expression has been immortalized in countless songs, from Lynyrd Skynyrd's 'Free Bird' to The Beatles' 'Blackbird,' cementing its place in popular culture.
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