Getting such a low mark on your test makes you a real bonehead.

你在测验中得了个这么低的分数,你真是个笨蛋.

That was a really bonehead move not to invite her friends along with them to the party.

真愚蠢到家了，竟然不邀请她女朋友的朋友和他们一道去参加聚会。