Jocelyn Davis
I don’t understand why “what” should be used here
Susan is what is known as an "addlehead"----she always leaves her key on the door. 

Why can’t it be “who is known as “?

Waiting for someone to explain.Many thanks.
Jun 2, 2020 6:16 AM
Comments · 1
What is known as = a thing or personality trait she exhibits.
Who is known as = what people refer to her as.
addlehead = muddled forgetful confused incapable of serious thought.

"Susan is what is known as an addle-head" = you are describing what is wrong with Susan what she is like, her problem, what to expect when you meet her.
"Susan who is known as addle head" = Susan is called addlehead by friends.
It can be 'who is known as addlehead' if that is what people call her.
June 2, 2020
Jocelyn Davis
Language Skills
Chinese (Mandarin), English, Japanese
Learning Language
English, Japanese