Obtuse in its literal sense means “blunt or dull,” and an obtuse angle is one of more than 90 degrees but less than 180 degrees. The figurative sense of obtuse (which is almost always applied to people and their behavior) is “dull, imperceptive, unintelligent, unthinking”: How can he be so obtuse as not to understand us?
Abstruse means “difficult to grasp or comprehend, needlessly complex, impossible
to understand” and is applied typically to arguments, propositions, and the like: Her proposals were so abstruse as to be meaningless to us. Avoid using obtuse as a rough synonym of abstruse.