verb/ˌno teˌneɾ ˌdos ˌdedos de ˈfɾente/
to be an idiot, to be thick as a plank
Literally, “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).