That means 'Irrational Human'. A play on Homo Sapiens - "wise human".
Here's a thought: maybe the defining feature of human beings is not our great thinking power, our 'rational' superiority to other animals. Perhaps our differentiating property is that of being the only animal to act irrationally.
After all - we seem quite able to describe all other animal behavior as part of a grand scheme to further the species, if not the individual. It seems that only humans act to the detriment of the individual and the species.
And I'm not just talking about acting on our emotions - the bane of economics and policemen alike. I'm talking about our tenacity in believing things like Freewill, the Self, God and the concept of "I" without being able to defend them philosophically. Despite the best efforts of the materialists and determinists out there, most people doggedly believe that they actually have the ability to control events, to make choices from among possible alternatives, etc.
How many other animals, once convinced that something is not possible, will keep trying it anyway?
btw - I found only one other relevant web citation of 'homo insensatus' - that for a 2005 Barnard College roundtable discussion.