1. Annoyingly insensitive or slow to understand.
2. Difficult to understand.  
Props to Dictionary.com for helping me with this blog post …
We’re all professional audience members, so we’ve heard the obtuse speaker before. That’s the one who leaves us thinking we missed something we probably should have known or picked up? Or we leave thinking s/he spoke in circles but never got anywhere? Or we’re frustrated because the s/he spoke over our heads and never bothered to find out or figure out if we were keeping up?
Obtuse speakers are egocentric, not audience-centric. When it’s your turn to speak, give more than just superficial thought to your audience. And if you don’t know enough about them ahead of time, throw (what you think is correct) protocol out the window and ask them … ! Yep, just ask. That’s right, either open, or take a pause, and solicit input from your audience to gauge their interest or knowledge levels. It will accomplish two things: 1. you will earn big engagement and connection points with your audience; and 2. you will avoid being obtuse. Thankfully.

Beth Levine