Her obtuseness in not understanding basic instructions frustrated her coworkers.

The angle's obtuseness meant that the slim acute would never love him.

Just kidding! Obtuseness means lack of intelligence or sensitivity, so:

Tim's obtuseness caused everyone to avoid him because he was dull and not fun to be around.

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Brute obtusness refers to the ocean. "faced by the brute obtuseness fo the ocean." It means that the ocean doesn't care for the boys, that it could care less that they're stranded on the island.

Quality of being impenetrable., That property in virtue of which two portions of matter can not at the same time occupy the same portion of space., Insusceptibility of intellectual or emotional impression; obtuseness; stupidity; coldness.