When a boy or girl is so full of themselves and thinks they are so fit and sexy or good looking but they are actually so ugly in general. They will try hard to look their best but it never works out. So it basically means a really ugly looking person who thinks they look good.
Millie: Hey did you see Bonnies post?
Lily: Yeah init she’s so sali.
Lili: Yeah isn’t it Lily.
Sofia: I hate people who are sali.