Obtuse is from the Latin, dull or blunt. It can describe an angle that is not acute- an angle larger than 90° but less than 180°.
When applied to 'an understanding' of something, it means that the person's comprehension is 'at a wide angle' to the topic. Literally, they are coming at the topic from a different angle, an angle larger than 90° but less than 180°. At ... a slant.
You might hear people say 'you are being deliberately obtuse' - it means that you are coming at the subject from a different or atypical perspective, a different angle, at a tangent, and it also can mean you are choosing not to understand the topic as others understand or perceive it (usually for your own purposes).
It really means 'choosing to interpret the situation in a different way from others' - often for one's own ends, to one's own benefit.
Example: when I was working at a large hotel in Biarritz, I was obtuse and I decided to misunderstand their requirement for their chambermaid staff to literally run from room to room tidying and throwing the duvets onto the beds in order to earn whatever the salary was, and I pretended that I didn't understand that I had to work like a slave there. When they came and said 'oh giddy up' I was like 'oh yes sure' but actually, I was being obtuse - choosing not to understand. They did of course ultimately sack me, but my lack of understanding of their 'speed up' request was me being deliberately obtuse. They said 'you don't understand the job in hand' but I was silently saying 'well actually I understand it perfectly, I'm just not doing it'.
By the way, a person is not 'obtuse' themselves. It is their understanding or response to a situation that is obtuse (or 'off at a wide angle, at a tangent').
By the way, you can say they are 'being obtuse' - about a topic, but not 'you are obtuse' - unless you are trying to say that they disagree with absolutely everything that exists.
The online dictionary definitions of obtuse are to me, confusing, as they are taken from contextual uses of obtuse, and they don't really define it in the same way, as if you think of it as 'coming at topics from a wide angle'.
For example, I found these (which I find spurious):
- not quick or alert in perception
- stupid and slow to understand
- lacking quickness or perception or intellect
Well yes maybe - if we are just coming directly from the original Latin meaning of 'dull'. But to me a more accurate way of thinking of it is 'off at an angle, off at a tangent - to the main argument or idea'.
When you call someone obtuse, I think the idea is to say that they are 'deliberately choosing a different or unusual point of view' - not that they're 'an idiot'.
Online you can also see 'if someone calls me obtuse, does it mean that they think I'm an idiot?' Well, I suppose it depends on 'how'!
(The answer is no, but it means they disagree with you, or have a more common point of view).
https://www.merriam-webster.com/dictionary/obtuse#:~:text=Obtuse%20comes%20from%20a%20Latin,with%20the%20similar%2Dsounding%20abstruse.