Stupid means lacking intelligence or good judgment, or showing poor reasoning. When someone makes a stupid mistake, they do something careless or thoughtless that they should have known better than to do, like leaving their bike unlocked outside in the rain or forgetting to study for a test they knew was coming.

The word often describes decisions or actions rather than people themselves. A smart person can still do something stupid by not thinking things through. Touching a hot stove after being warned it's hot would be a stupid thing to do, not because the person lacks intelligence, but because they ignored obvious information.

Stupid is also used more casually to mean annoying or frustrating, like when someone complains about a stupid rule or a stupid rainy day that ruined their plans. In this sense, it just means unwelcome or bothersome.

Calling another person stupid is hurtful and usually unfair. Everyone makes poor decisions sometimes, has gaps in their knowledge, or struggles with certain subjects. The word works much better when describing specific choices or situations rather than people. Instead of saying someone is stupid, it's more accurate and kinder to say they did something unwise or made a poor choice.