The word "sickie" is a colloquial term used to describe a day taken off work due to illness. However, there are several synonyms that can be used in place of this informal word. These include "sick day," "illness absence," "medical leave," and "health-related absence." Each of these terms conveys the same meaning as "sickie," but in a more professional and appropriate manner. It is important to use proper language in the workplace, and avoiding informal language like "sickie" can help maintain a professional tone and impression.