obtuse

The adjective obtuse is good for describing someone slow on the uptake: "Don't be so obtuse: get with the program!"

The adjective obtuse literally means "rounded" or "blunt," but when it's used for a person, it means "not quick or alert in perception" — in other words, not the sharpest tool in the shed. It's not just for dull people, but also dull angles: in geometry, an obtuse angle is one that is not so sharp (between 90 and 180 degrees).

Definitions of obtuse
  1. adjective
    of an angle; between 90 and 180 degrees
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    Antonyms:
    acute
    of an angle; less than 90 degrees
  2. adjective
    lacking in insight or discernment
    “too obtuse to grasp the implications of his behavior”
    synonyms: purblind
    undiscerning
    lacking discernment
  3. adjective
    slow to learn or understand; lacking intellectual acuity
    “he was either normally stupid or being deliberately obtuse”
    synonyms: dense, dim, dull, dumb, slow
    stupid
    lacking or marked by lack of intellectual acuity
  4. adjective
    (of a leaf shape) rounded at the apex
    Synonyms:
    simple, unsubdivided
    (botany) of leaf shapes; of leaves having no divisions or subdivisions
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