When set loose I fly away, Never so cursed As when I go astray.

Common Wrong Answers
“A balloon”
While a balloon can fly away when released, it is not typically associated with the idea of going 'astray' in a cursed sense, nor does it have the same connotation of being a natural bodily function.
“A bird”
A bird does indeed fly away, but it does not fit the context of being 'cursed' or being something that can be controlled and released from the body like a fart.
“A kite”
A kite can fly away and requires wind to soar, but it doesn't have the same implications of being a natural release or the idea of embarrassment that comes with a fart.
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