The literal meaning is " the one who knows" , but now it is used as " an acquaintance"

Dunno that I agree with that one. I've only heard it used to refer to those "in the know," meaning experts, authorities in a field, "mavens." Perhaps with a sense of inflated self-importance thrown in. The cognoscenti are a cut above hoi polloi.

Having it mean "acquaintance" is a totally brand-new usage to me. Maybe I'm too much into Gilbert and Sullivan...