noun
parting shot; sharp parting remark; parting threat
A cutting remark or threat thrown at someone as one leaves, often in arguments or dramatic exits. Common in fiction and everyday quarrels.
彼は「もう二度と来るな」と捨て台詞を残して出ていった。
He left with a parting shot: "Don't ever come back."
口論の末、彼女は捨て台詞を吐いて部屋を
After the argument, she flung a sharp remark and stormed out of the room.
Compound of 捨てる (to throw away) and 台詞 (lines, speech). Literally 'throw-away lines', referring to words cast at someone as one departs.