This is the 505th most frequent German word.
einst
"Once" or "formerly" (referring to the past).
'Einst' is used here to mean 'once' or 'at one time', referring to a point in the past.
Einst war dieser Baum der höchste im ganzen Wald.
Once, this tree was the tallest in the entire forest.
In this sentence, 'einst' means 'one day' or 'someday', referring to an undefined time in the future.
Die Reiche der Welt werden einst untergehen.
The empires of the world will someday fall.
'Einst' in this context again refers to 'once' in the past, specifically tied to a personal memory or observation.