The ‘ginger nut’ biscuit is the origination of this redhead nickname. Golden in colour, sweet and spicy in flavour, these biscuits are similar to redheads in both sight and taste (we like to think so, anyway).
Also, literally translated, ‘ginger nut’ could mean: ‘ginger’ = red, ‘nut’ = head.
Either that, or ‘ginger nut’ is being used to describe an eccentric redhead. Sometimes it’s hard to know which.