can be used as an insult, but sometimes saying someone is stupid can be a compliment. for example, trump is so stupid is actually a compliment because we all know that trump is the stupidest out of all the stupids that exist.
i am stupid , you are stupid, but trump is way more stupid that us
What you feel after you do something totally unnecessary, but you need to do it anyway and something you didn't expect happens, and it's only afterwards that you realise what you've done. Usually involves a person of the opposite sex and/or someone you got a crush on.
Liam thought he would say hi to the girl he liked, but Liam unintentionally freaked out said girl. Now the girl wont talk to him. Liam feels stupid.