A D.A.B. is a girl who is a "drunk ass bitch". She often knows and may refer to herself as a D.A.B. They are often very hammered and do not think before acting, therefore they look silly.
A designated D.A.B. is the person for the night who will become the most shit faced and act the most ridiculous. They can be appointed at the start of the night, but most often as the night goes on, the designated D.A.B. slowly surfaces.
That girl is dancing off beat to the music and is spilling her drink everywhere. She looks like she might be the designated D.A.B. for the night.