almost like silly

少根筋 literally means to have "one muscle fiber too few (in the brain)," but colloquially it is used to refer to someone who is forgetful or not very observant. For example, a person who often forgets his keys, or a guy who is not aware that a girl likes him (when everyone else is aware of the fact) can be said to be 少根筋.

thanks so much

well,it`s ignorant....thinking about everthing too simple.