l and f
An L and F is someone who is overly protective of their friends and will get mad if you make a joke. They take everything seriously, even if you're just messing around.
"Don’t joke about my friend’s hair. I’m an L and F.", she was mad because he said her friend’s hair was ugly.
"He’s an L and F. Don’t say anything funny.", his friend was being serious and didn’t want to laugh.
"She’s an L and F. Don’t make her laugh.", he was trying to be funny and failed.