If someone is straightlaced, they are very strict and proper, and they don't like to do things that are not proper.
Being straightlaced can make people think of old-fashioned or uptight people.
Examples:
She is very straightlaced and doesn't like to have fun.
My teacher is straightlaced and doesn't like jokes.
He is too straightlaced to play with us.
The straightlaced man didn't like the silly dancing.
The straightlaced lady never smiles or laughs.
The straightlaced lady always wears very proper clothes.