他脑子少了一根筋怎么翻译

He's one short of a six-pack.（俚语）

啤酒六听一包，少了一听，比喻人少根筋。
A term used to describe a person who is not all there. A person missing a marble or two - a few percentages off from being full blown retarded.

He's not wise enough.

His the brain misses a sinew.
OK?^_^