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 can't believe I'm doing this. Should we stop? We could just go for a run. I don't know, are we getting too old for this? - Sex? - No, casual sex. It just, I don't know, feels a little collegey. - I could sing some Third Eye Blind. - Okay. - That's not Third Eye Blind. (..)2011 Friends with Benefits