！
That's my line!
Literal
That [topic-は] my [genitive-の] line [copula-だ] [emphasis-よ]!
A common comeback when someone says something the speaker was about to say. 台詞 (せりふ, 'serifu') literally means 'lines (in a play)' — the words an actor speaks. Used metaphorically, it refers to anything that 'belongs' to a particular speaker as their characteristic phrasing. The exclamatory よ adds emphasis to the protest.