Mesh jacket, if you have one.
Worse comes to worse, any light jacket is better than just wearing a t-shirt.
Whatever additional layer of material you add over the t shirt, helps.
In a minor slide, even a light, non-motorcycle jacket helps a bit.
Plus...you have bugs, flying gravel and sand to deal with on bare arms.
If you're going to ride at 'highway' speeds, it's not worth it....