A slang phrase, stemming from the identical phrase regarding the workplace, characterizing a situation as purely or primarily platonic in order to keep things simple, without complications from casual or serious romance.
Girl: I’m definitely down to take a trip with you, but we should keep it business casual. We’ve been just friends for a really long time and I wouldn’t want to ruin that.
Guy: Yeah that’s smart. Hooking up would make things too complicated.