Obtuse usually describes a person who is slow to grasp something obvious, or, in geometry, an angle wider than 90 degrees. Abstruse describes something itself — usually an idea, subject or piece of writing — that is genuinely difficult to understand. The quickest test: obtuse points at a person's understanding, abstruse points at a subject's difficulty.
The core difference
Obtuse describes someone failing to understand something (or a blunt angle or shape), while abstruse describes a subject that is hard to understand.
- obtuse — slow to understand what seems obvious to others; also a blunt angle: He was being deliberately obtuse, pretending not to see the point.
- abstruse — difficult to comprehend; obscure: The lecture drifted into abstruse points of quantum theory that lost most of the room.
How to tell them apart
Obtuse comes from Latin obtusus, "blunt or dull," from a verb meaning to beat something blunt — which is why it also names a wide, blunt angle in geometry, and by extension a person whose understanding feels blunted or dulled. Abstruse comes from a completely different root, Latin abstrudere, "to push away and hide," so it describes something that has effectively been hidden from easy understanding.
A memory hook: an obtuse angle is a blunt, wide angle — think of an obtuse person as similarly "blunt" to a point everyone else sees. Abstruse material is the kind that's been "tucked away" behind jargon or complexity, matching its root sense of concealment.
