Someone who acts like they own the place and that the whole world revolves around them. They look after number one, and everyone else is low on the priority list. If you know the arsehole well, often even just their presence can make you feel small and insignificant, and generally they treat you like something they stepped on.
He just strolled in, ignored the shop assistant, took what he wanted, threw the money on the counter and left. What an arsehole.