ถ้อยคำฝู (福)fu福

The custom is old enough to date: mounting the character goes back at least to the Song dynasty, 960 to 1279 CE.

The upside-down part is a pun, and the pun is the mechanism. In nearly all varieties of Chinese the word for upside-down and the word for to arrive are homophones — dào, written 倒 and 到. 'Upside-down fu' therefore sounds all but identical to 'good luck arrives', so pasting the character inverted on a door or doorpost states a wish that prosperity descends on the house.

A second story explains the same thing differently. It puts the origin with the family of a nineteenth-century Qing prince: on a New Year's Eve the prince's servants pasted fu characters through the house as a joke, one illiterate servant put his on upside down, and when the prince was furious another servant saved him by pointing out that the mistake announced prosperity arriving. Two accounts, one practice; the encyclopedia carries both without choosing.

The sound does further work. Bats — 蝠 — are among the most common of Chinese symbols and carry the same sound as fu, which is why textiles and ceramics show bats in flight, sometimes more than a hundred at once. Bran, 麩子, fūzi, is a rarer match.

The other fu is a different trade. In Taoist practice, 符 are instructions to deities and spirits, symbols for exorcism, and recipes for potions and cures, written by priests of the fulu pai. Their illegibility is not a failure: Ge Hong noted in the Baopuzi, by the Eastern Jin dynasty of 317 to 420, that successful use did not depend on the user's being able to read the script, and scholars including James Robson and Gil Raz argue the incomprehensibility is central to the talisman's authority.

The fortune character now has a code point: Unicode 10, in 2017, added a rounded version at U+1F260 🉠 ROUNDED SYMBOL FOR FU.