What is this purple briuse on my arm?

Blood. Blood thinners? Aspirin ? Plavix ! Coumadin (warfarin) ? If there is no history of blood thinners, this is a bruise see more commonly in people with thin skin or advanced age, suggest complete blood count at next examination .
Actinic purpura. As we age, the skin on our arms thins in response to sun exposure. The veins within that thinned skin are now relatively unprotected, and break easily when scratched or slept on wrong, causing bruising. This is called actinic purpura, and the bruises resolve by themselves without specific treatment. That is the most common cause of unexplained bruising on the arms.