Relationship between Nasus and Sivir (Before and after the lore update)

Before Sivir even had a remodel (yes, back when she had pizza feet), she was friends with Nasus. Even now, Nasus and Sivir share a friendlly relationship. From a character who went from being just 'Battle Mistress' to a tomb robber, I don't suppose anyone can explain to me how they even met (let alone, explain why they're friends in the first place). This isn't just me being a smartass or anything, but just me being curious of the whole thing.