One sandwich short of a picnic:脑袋少根筋,愚笨
One sandwich short of a picnic:意思是脑袋少根筋,愚笨.笔者也见过'two sandwiches short of a picnic'的说法,但意思都一样.Generally used when telling someone they are really stupid.
类似'an X short of a Y'这样的短语还有很多,都是同样的意思,例如还可以说成one brick short of a load,a few fish short of a hatstand等等.
After talking to him for about 10 minutes I decided he was definitely one sandwich short of a picnic. 我和他谈了10分钟话,觉得他很笨(脑袋缺根筋).
You really are one sandwich short of a picnic aren't you! 你弱智啊?(慎用)