going, from
于 centers on movement or direction: going somewhere or coming from a place. Its meanings are primarily grammatical or directional, indicating motion or origin.
于 is an ancient character whose original pictographic meaning is uncertain. It may have depicted a curved object or a breath, but its use as a grammatical particle for direction or location developed early. The modern form is simple and abstract.
The two horizontal lines suggest a starting point and a destination, with the vertical hook showing the path going from one to the other.
For ウ, imagine a person saying 'Ooh!' as they set off on a journey: 'Ooh' -> ウ, and they are going from here to there.