What does Boxing Day have to do with boxing?
1950, with -ism + name of U.S. Sen. Joseph R. McCarthy (1908-1957), leader of U.S. anti-Communist agitation. The term is said to have been coined by "Washington Post" political cartoonist Herbert Block ("Herblock"). The surname is from Irish Mac Carthaigh "son of Carthach" (Welsh Caradawc), an ancient Celtic name, also known in its Latinized form, Caractacus (last of the British leaders to resist Rome, captured 51 C.E.)
Note: McCarthyism has become a general term for the hysterical investigation of a government's opponents or the publicizing of accusations against these opponents without sufficient evidence to support the charges.