Ass Paper is commonly referred to as "toilet paper", or "bath tissue". There is no difference. However, the term "Ass Paper" is actually the correct word to describe what it's used for. Toilet paper can be misleading, some can assume you use toilet paper to clean the toilet. Bath tissue can be used to clean the bathtub. Ass Paper can only be used to clean your asshole after you take a shit.
A sunburn or suntan focused on the left arm. It is caused by the left side of the body being the only half that gets exposed to sunlight while driving. Presumably, it would be located on the right side of the body in countries where they drive on the left side of the road.
British slang for a scientist or technical expert, esp . when the speaker is not certain of the expert’s specific field of study. Not necessarily derogatory, but typically used by persons without a technical background. In the US, the equivalent would be “egghead”, “squint”, or “geek.”
Origin of the word is unknown. Started sometime in World War 2. Given the typical evolution of British slang, could easily have started because what these experts did was confusing or “baffling” to the common person.