The Chinese character pasted upside down to mean good luck has arrived is Fu, written 福.
Fu means “good fortune,” “luck,” or “blessing,” and it is one of the most recognizable decorations of the Spring Festival. It is commonly written in black or gold on a square red paper, then placed on doors, walls, windows, or household objects.
The upside-down custom depends on a Mandarin wordplay. The expression for “upside down” is dao (倒), while “to arrive” is also pronounced dao (到). Thus, “upside-down Fu” can be heard as “fortune arrives,” turning an unusual placement into an auspicious message.
Fu is often confused with other popular New Year characters. Lu traditionally represents prosperity or status, Xi means happiness, and Shou is associated with longevity. Unlike those characters, Fu is specifically the one linked to the famous inverted-door pun. In some decorative contexts, however, Fu may be displayed upright as well.