。
You've got to be kidding me.
Literal
Joke is-not [よ].
冗談じゃない is a fixed idiom for 'you've got to be kidding / this is no joke / no way.' ねえ is the rough masculine contraction of ない, making this a gruff masculine rendering. The final よ is the assertive particle. A three-word exclamation, maximally concise Japanese frustration — perfect for anime/manga dialogue.