They are only 'casual' acquaintances. [=they know each other but not very well]
She takes a 'casual' [='nonchalant'] approach to her homework.
He was dressed 'casually' in jeans and a sweatshirt.
(..) I'm getting a nice soap-and-water feeling from down there. A female. "Female"? You callin' her female must mean you like her or you wouldn't be so casual. (CHUCKLES) Is she alone? Yeah, she's alone. Things are heatin' up. Chestnut hair? (..)1992 Scent of a Woman