"I don't believe you!"

• Marx film shot at Eider Downs?
• Marx Brothers classic
• Poppycock
• Proof of the missing link between equines and birds?
• Bunk

Douglas Harper's Etymology Dictionary Word definitions in Douglas Harper's Etymology Dictionary
"nonsense," 1928, said to have been coined by U.S. cartoonist Billy De Beck; perhaps a variant of horseshit "nonsense," though the latter is attested only from 1940s.

Wiktionary Word definitions in Wiktionary
interj. (context euphemistic English) Indicates disbelief. n. 1 (context euphemistic English) nonsense; indicates disbelief. 2 Long hair on lower legs of a draft horse (e.g., Clydesdale), especially the rear legs. 3 feathering strip.