Nonsense; rubbish.
Of unknown origin; the word was popularized by the Irish-American cartoonist Thomas Aloysius (“Tad”) Dorgan (1877–1929), who started using it in cartoons on March 9, 1922. Maybe from Irish mallachtóireacht or Greek μαλακίες (malakíes)
Practice speaking and memorizing "malarkey" and many other words and sentences in English.