Half in jest; partly joking but with some seriousness or possibility behind it.
冗談半分で宝くじを買ったら、少し当たって驚いた。
I bought a lottery ticket half as a joke and was surprised to win a little.
あの話は冗談半分に聞いておいて、本気にしないでください。
Take that story as half a joke and don't take it seriously.
彼のプロポーズは冗談半分だったが、後で本気だとわかった。
His proposal was half in jest, but later it turned out he was serious.