Seriously; no kidding. Used to indicate that one is speaking earnestly or putting jokes aside.
冗談抜きで言うけど、君の提案には賛成だ。
Seriously, I agree with your proposal.
冗談抜きで聞くが、転職を考えているのか？
I'm asking seriously—are you thinking about changing jobs?
冗談抜きで、これをやらないとまずいよ。
No kidding—you really need to do this or it'll be bad.