Keeper of the Superfluous Es!

I have assumed that at sometime when younger (possibly even pre-Hogwarts), at least one of the Mauraders reduced Snape to tears. That said nicknames, based around the first name and/or surname is a typically British schoolboy thing to do and they don't even have to reflect personality traits. my mother was known as Loony (from Loony Bin because her name was Lunan), I had a classmate to was know as Ena (from Ena Sharples, a gossipy tv character, because his surname was Sharp).