The characters literally combine to mean “ten thousand (countless) years of life without a boundary.” This creates a powerful, poetic image of a life that extends infinitely, unconstrained by limits.
Historically, `万寿无疆` was a phrase reserved for the emperor of China. It was a wish for his eternal life, which by extension meant the eternal stability and prosperity of the empire. Chanting this was a demonstration of ultimate loyalty and submission to the “Son of Heaven.” A good Western comparison is the phrase “Long live the King/Queen!” However, there is a key difference in modern usage. While “Long live the Queen!” is still exclusively tied to a monarch, `万寿无疆` has evolved. After the fall of the imperial system, this grand blessing was repurposed to honor the most respected figures in a family: the elders. It is now a cornerstone of expressing filial piety (孝顺, xiàoshùn), the Confucian virtue of respect, obedience, and care for one's parents and elderly family members. Using this phrase for a grandparent on their 80th birthday is not just a simple “happy birthday.” It is a profound cultural act that elevates them to a status of utmost respect, acknowledging their wisdom and their central role in the family, much like an emperor was to the nation.
`万寿无疆` is extremely formal and its use is highly context-dependent.
It is crucial to understand that you would never use this phrase for friends, colleagues, or anyone younger. Saying `万寿无疆` to a friend on their 30th birthday would be bizarre and might even be interpreted as a joke or sarcasm, as if you're calling them ancient.