When you say something is a "piece of cake," it means it is very easy to do.
In the 19th and early 20th centuries, African Americans started "cake walks." They walked or danced around a cake in pairs, and the best couple won the cake. This is how the idioms "cake walk" and "piece of cake" came about -- both meaning that something is very easy to do.
A: Hey! How did you do on the exam?
B: I got an A. The exam was a piece of cake.
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.