say
云 centers on the act of speaking or saying something. It is a simple verb for verbal expression.
云 is a pictograph of breath or vapor rising, which came to represent speaking. The modern form retains the basic shape of a rising cloud-like form.
The top 二 looks like two lines of speech, and the bottom 厶 resembles a mouth or a person speaking. Together, 云 is the act of saying something.
For ウン, imagine a person saying 'un' as a thoughtful sound before speaking: 'un' -> ウン, and then they say their words.