From Scottish/Irish Gaelic Mac Artair meaning 'son of Arthur', itself from a Celtic root meaning 'bear man'.
Macarthur — or MacArthur — is a Scottish and Irish surname pressed into service as a given name, following the long tradition of honoring great men by naming children after them. The name is a Gaelic patronymic: Mac Artúir, meaning "son of Arthur." Arthur itself is of debated origin — possibly from the Celtic artos ("bear"), the Roman family name Artorius, or the Welsh arth ("bear") combined with viros ("man") — and it is of course the name of Britain's legendary king, making MacArthur literally "son of the legendary warrior king."
The clan MacArthur is one of the oldest in Scotland, with a proverb claiming it predates all others in Argyll. As a given name, Macarthur is most powerfully associated with General Douglas MacArthur (1880–1964), one of the most decorated and controversial American military commanders in history. His leadership in the Pacific during World War II and his famous "I shall return" vow made him a national hero; his later relief from command by President Truman during the Korean War made him a symbol of the tensions between military and civilian authority.
Parents who named sons Macarthur in the 1940s and 1950s were almost certainly honoring the general. As a given name today, Macarthur is extremely rare and carries unmistakable mid-century American patriotism. It reads as bold and historical — a name with weight and biography built in — suited to someone who carries it with the quiet confidence that a name that big demands.