When one drives a vehicle a lot with the window rolled down and thier arm resting on the edge of the door where the window is rolled down - that arm receives a lot more exposure to the sunlight than the other arm, resulting in that arm being more tanned than the other.
Because the occupation of a taxi driver requires a lot of driving, sometimes they get the "taxi driver's tan" effect.
Bob: Hey look at John in his tank top.
Joe: Wow, look at his arms, he sure has a taxi driver's tan.
1) A sun tan that extends to the elbow of one arm (the offside arm), frequently displayed by taxi drivers who nonchalantly steer one handed with the other grabbing the roof, while looking in the mirror to gauge the reactions of their passenger to their reactionary opinions.
2) A tanned appearance of one finger, the result of a catastophic failure in the integrity of the toilet paper when wiping the anus.
"Sod this hotel, the water's off, the bog roll's damp and I've got a taxi driver's tan"
A term used when the finger breaks through the toilet roll during wiping, causing the finger to be coated in faeces. Derived from the way taxi drivers put their arm out the window and hold on to the roof with their fingers, therefore getting a tan on their fingers. See also: 'Breaching the hull'
I was wiping my arse this morning, but I breached the hull and got a taxi driver's tan.