"Miffy"

[Koromo]

I know a Miffy. Her surname is Smith.

Smith --> Mith --> Miff --> Miffy

Well, that's how she explains it. 

[pjbuk007]

I reckon the Miffy comes from Smith in this case.

But some names come from childhood nicknames which stick/mispronounciations by siblings.  Nona for Iona is one example.  My grandmother was Adeline, a great name, but my grandfather always called her Mary, so everyone else did.

[sueky71]

nicknames/petnames are so strange.
i never found out why my dad called both me and my sister.......tilly.
neither of our names (sue & yvonne) had the slightest connection to either each other or tilly.
also if he shouted tilly and the 'wrong' one answered he'd say 'not you, the other one'!