a few, two, etc. bricks short of a load
a few, two, etc. ˌbricks short of a ˈload(also a few, two, etc. ˌsandwiches short of a ˈpicnic) (informal) (of a person) not very intelligent; slightly crazy: If you ask me, I think he must be one or two sandwiches short of a picnic! OPPOSITE: all there
The structure a few, two, etc....short of a(n)... is often used to make new idioms, for example a few cards short of a deck, a few letters short of an alphabet.