I contend that what most of world calls being stupid is more likely to be considered "being inconsiderate" and I believe that the definitions will bear me out.
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Stupid - Definition and More from the Free Merriam-Webster Dictionary
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Definition of STUPID
1 a : slow of mind : OBTUSE
b :
given to unintelligent decisions or acts : acting in an unintelligent or careless manner
c : lacking intelligence or reason : BRUTISH
2 : dulled in feeling or sensation : TORPID <still stupid from the sedative>
3 : marked by or resulting from unreasoned thinking or acting : SENSELESS <a stupid decision>
4 : lacking interest or point <a stupid event>
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Inconsiderate - Definition and More from the Free Merriam-Webster Dictionary
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Definition of INCONSIDERATE
1 a : HEEDLESS, THOUGHTLESS b : careless of the rights or feelings of others
2 : not adequately considered : ILL-ADVISED
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Would the older driver been more considerate of your situation or did he not adequately consider the ramification of his actions at the time.
After all you did have your turn signal on, announcing your intentions to turn right. He can turn left in front of you legally as long as he doesn't create a hazardous condition for traffic flow. That being said one needs to consider all the possibilities and how can one do that without being well informed of your driving habits before had. I contend his decision to turn in front of you was ill advised even though lawful since everything could not be considered properly.