The word "antonym for birthday" refers to a term that signifies the opposite of a birthday. While a birthday is a celebration of the anniversary of a person's birth, an antonym could represent an event that signifies the end of life or a significant moment related to death.
The term "birthday" comes from the combination of "birth," which originates from the Old English "berthan" meaning "to be born," and "day," derived from the Old English "dæg," meaning "day."