aka I LEARNT A THING AT EDINBURGH CASTLE
So, apparently because the 'pan' of a musket was so close to the face of whoever was firing it (the 'pan' being the bit of the gun where you put the gunpowder - hence 'flash in the pan'), it was often common for a soldier's face to get slightly burnt after continual use. The solution that many soldiers came up with for this was to grow the facial hair on the sides of their face long, so as to avoid getting burnt. This is where the word 'sideburns' comes from!
On an unrelated note, would any here care to guess what a 'jiggarabbit' is? A scouser at Blue Planet told me but I'm not sure I believe him.