Zelda comes from the Yiddish Zelde, derived from the Germanic sælig, meaning "blessed" or "happy." The same root gives the English "silly" (originally meaning "blessed" before its meaning shifted).
Zelda Fitzgerald (1900-1948), the writer and wife of F. Scott Fitzgerald, brought the name into American cultural memory. The Nintendo video game series The Legend of Zelda (1986 onward), in which the princess Zelda is a recurring central figure, gave it a second life. Today Zelda sits in the U.S. top 600 and is rising fast.
Zelda reduces to four — the number of blessed steadiness.