Definition of the word rear end

The phrase "rear end" originally referred to the backside of an animal, specifically a cattle's or horse's backside. The term gained popularity in the late 19th century in the context of transportation, particularly with the increasing use of stagecoaches and railway cars. As these forms of transportation involved passengers sitting facing one another, the backside of one passenger could be visible to others. Hence, the term "rear end" came to describe this anatomical part of another person that could unwittingly be exposed to others' view. Over time, the term has evolved to be used more figuratively to refer to any embarrassing or untoward situation or problem that arises from behind, such as in computer systems or mechanical equipment.