Rare, literary term for a letter, often with a classical or poetic feel. The kanji literally mean 'wild goose's writing', referencing the ancient Chinese legend of a letter tied to a goose's leg.
The word 雁の文 is sometimes used as an elegant term for old-fashioned letters.
From the kanji 雁 (wild goose) and 文 (writing, letter). The expression originates from a Chinese legend in which a letter was tied to the leg of a wild goose, making the goose a messenger. The exact historical derivation into Japanese is uncertain, but the term is used as a poetic or classical reference to a letter.