urgent letter; express letter
An old-fashioned term for a letter sent with urgency, similar to 急便 or 速達 in modern Japanese. Rarely used today.
He wrote an urgent letter and sent it by fast horse.
Compound of 飛 (to fly, implying speed) and 札 (a tag or letter). The exact historical derivation is uncertain, but the kanji suggest a message sent swiftly.