This is the 5674th most frequent Japanese word.
すん
"すん" is an old Japanese unit of length, approximately 3.03 centimeters.
In this sentence, "すん" is used as a representation of the colloquial contraction of "すむ" (to finish or to end).
もうすんだの？
Have you already finished?
Here, "すん" is used to express purity or clearness, derived from "澄む" (to become clear or to be pure).
これはすんだ水です。
This is pure water.
In this usage, "すん" is part of the verb "済む" indicating completion or conclusion of an action.