as opposed to bullshit, which expresses judgement on shallow, insensible, and false ideas, horseshit puts more emphasis on the speaker's distress about the message just received
From "Where's Poppa?" (1971)
Mrs Hocheiser (senile, and seemingly annoyed at the presence of Louise Callan, a nurse to take care of her): "Who is she?"
Gordon Hocheiser (to his mother): "Mom, she's here to help us."(silently) "I'm warning you, mom."
Louise (pushing Gordon to the kitchen) "Gordon, you just get things started! (to Mrs Hocheiser): "We're going to just have a nice evening!"
Mrs. Hocheiser (asides, silently) "Horseshit ...