one-eighty

The term "one-eighty" is a modern, informal compound derived from the numerical phrase "one hundred and eighty," specifically referencing the 180-degree turn. Its logic is purely geometric and metaphorical, originating from the domain of physical rotation—where turning 180 degrees leaves one facing the exact opposite direction. This literal sense of a complete directional reversal was then abstracted into the figurative realm to describe a total change in opinion, policy, or stance. The truncation to "one-eighty" follows a common pattern in colloquial English where longer numerical phrases are shortened for brevity and impact, solidifying its place in idiomatic usage.

Structure: one (numeral) + eighty (numeral) one: From Old English 'ān', meaning the number one. In this compound, it represents the first part of the numerical value 180. eighty: From Old English 'eahtatig', meaning the number eighty. In this compound, it represents the second part of the numerical value 180.