Unnecessary expenses are costs that are not essential for a person or business to function. They are often considered to be wasteful and can lead to financial hardship. The term 'FAT' is an acronym for 'Fixed and Variable Total', which is a way of categorizing expenses into two categories: fixed and variable. Fixed expenses are those that remain the same regardless of the amount of activity, while variable expenses change depending on the level of activity. Unnecessary expenses can be either fixed or variable, but they are not necessary for a person or business to function.