What does up mean when serving a mixed drink?

The term "up" in bartending refers to a drink that is served without any ice. This is in contrast to "on the rocks," which refers to a drink that is served with ice. Drinks that are served up are typically those that are meant to be enjoyed neat, such as many neat alcohols, or those that are made with ingredients that would melt or dilute quickly in the presence of ice, like juices or cream.