In England, a nappy is a noun meaning diaper. In white American baby talk, a nappy is a brief sleep. But in Black America, nappy is an adjective describing the tightly curled hair characteristic of black people.
Colloquially, it often seems to mean wild or unkempt hair - or hair that needs a trimming. As in:
"My hair is getting nappy."
As jordanzimmerman reminded me, Stevie Wonder sang:
Looking back on when I
Was a little nappy headed boy
Then my only worry
Was for Christmas what would be my toy
There's a children's book called Nappy Hair, written by a black woman. The idea is that "nappy hair" is a perfectly good kind of hair. But a white teacher got in trouble for trying to use the book in the classroom.
So, it means black people's hair, and it's often used negatively, even among black people.
So, if you're white, and want to stay happy
I suggest you best beware
Of daring to use this loaded word "nappy"
About your black friends' hair.
But if you really want to feel crappy
Say it on the air.