"Fat-Brained" is a word in ENGLISH

The faculty by which ideas are conceived; understanding; as, a man of dull apprehension.

Not well advanced in learning; not quick of apprehension; dull; inapt; as, a backward child.

Of obscure vision; not seeing clearly; hence, dull of apprehension; of weak perception; obtuse.

Slow of understanding; wanting readiness of apprehension; stupid; doltish; blockish.

Dull of apprehension; not possessing quick intelligence.