adj. stu·pid·er, stu·pid·est
1. Slow to learn or understand; obtuse.
2. Tending to make poor decisions or careless mistakes.
3. Marked by a lack of intelligence or care; foolish or careless: a stupid mistake.
4. Dazed, stunned, or stupefied.
5. Pointless; worthless: a stupid job.
Interestingly this word derives from both the French stupide as well as Latin stupidus, from stupre, to be stunned, from stupēre to be amazed.
How it comes to mean slow to learn or understand I am not sure. I think that meaning is a more modern meaning applied to people with low intelligence or slow on the uptake of something but its original roots meant more like the subject was amazed at what he/she saw.