no tener dos dedos de frente

Etymology
lit. 'not to have a two finger forehead'.

"Dos dedos" (Two fingers) is an unofficial measuring unit, roughly 3 to 4 cm measuring one finger on top of the other, and it is used to mean that someone is not very smart, as shown by displaying a forehead that only measures 3 to 4 cm high(two fingers).

Verb

 * 1)  to be an idiot, to be thick as a plank