Abstruse

abstruse \əb-ˈstrüs, ab-\ adjective: difficult to comprehend; recondite {the abstruse calculations of mathematicians}

Etymology: Latin abstrusus, from past participle of abstrudere to conceal, from abs-, ab- from + trudere to push

Try pronouncing the word abstruse. It isn't very round or open, is it? It almost feels as if you're extruding the word out of your oral cavity.