Snap Someone's Head Off Meaning
(idiomatic) To suddenly and sharply rebuke or insult a person, especially in response to a harmless remark.
Example: 1915, Kathleen Norris, The Story of Julia Page, ch. 5,
"Pa's awfully cranky," Mrs. Cox said resignedly. "He's always been that way! You cook him corn beefâ€”that's the night he wanted pork chops; sometimes he'll snap your head off if you speak."
1918, Kate Douglas Wiggin, The Story Of Waitstill Baxter, ch. 8,
He won't have a pleasant morning, I can tell you! I shall snap his head off every time he speaks to me.