Lost words are commonly used words that have wandered away from their original meaning, often in an amusing direction, and where this original meaning has become completely lost. For reasons that are a matter for conjecture, words travel more freely in the USA.
Pedants enjoy keeping an eye on the peregrination of lost words. Sometimes they attempt to haul the prodigals home again. Eventually, however, even pedants have to accept that the meaning has changed, though they rarely do this with good grace.
Then: Reduce to nine tenths of a previous number.
Now: Destroy completely, or reduce to one tenth of a previous number.
Status: Lost, but could still be rescued.
When a Roman legion, or regiment, exhibited cowardice in battle then the punishment would be decimation. This required one in every ten men, chosen by lot, to be executed. This was an extremely severe punishment, but could sometimes be carried out without breaking military discipline on a sufficiently shamed legion.
Then: One thousand, the number of soldiers in a Roman regiment.
Now: A huge, uncountable number.
Status: Wandering in the underbrush at the edge of a trackless forest where a legion trees darken the sky.
Then: A million million.
Now: A thousand million.
Status: Crossed the pond, became severely reduced by the privations of the trip, and never returned.
A billion is an English word meaning a million million, but the American version is a thousand million. Britain has recently adopted the American version.
Status: Missing in action.
Then: Happy and carefree.
Status: Socially irretrievable.
This is a relatively recent change, causing many embarrassing (but amusing) mistakes.