When you 'cock a snook' (the spread hand with thumb on the nose. It is called 'Five fingered-salute' in the USA. ) at somebody, you make fun of the person or you take a piss out of him. It is said that the history of this phrase can be back to somewhere in the nineteenth century when it became widely known in the UK.
Snooking devil mascots were often seen on pre WW2 machines at their back. The reason why the devil is facing backwards is because to cock a snook at the machine you have just beaten! There are some different type of designs but the one we chose this time has a big belly and we think that chubby shape was formed by drinking tons of bitter beers! (Very British, huh?) And we also dressed him like a gentleman with winkle pickers, which started to be worn by rock'n'roll lovers in the 50's.
Although we are supposed to be grown ups and gentlemen, we are still (big) boys and the cheekiness and naughtiness will never change!