Kennedy comes from the Irish surname Ó Cinnéide, meaning "descendant of Cinnéide" — where Cinnéide is from cenn (head) and éide (armour, helmet) — translating roughly as "helmeted head" or "helmeted chief."
John F. Kennedy, the 35th U.S. President, gave the surname its modern resonance. As a girl's first name, Kennedy entered the U.S. top 1000 in 1994 and now sits in the top 90 — used about 95% for girls.
Kennedy reduces to three — the number of confident expression.