An item of clothing that is often worn by surfers, but is increasingly worn by non-surfers. The name comes from the way that the shirt helps prevent a rash forming on the chest of the surfer caused by lying on the board.
Rash shirts are typically made of a combination of nylon and lycra, and are worn tight against the skin. They can be long- or short-sleeved.
In addition to their rash prevention characteristics, they also protect from sunburn, which explains their increasing popularity amongst non-surfers, particularly in Australia, which has a preponderance of light-skinned people and very high levels of UV radiation.
Bizarrely enough, the wearing of rash shirts is banned in some Japanese outdoor pools. Damn those pool nazis!