Readings & Meanings:
to soil, to get dirty
Air pollution is a serious environmental problem.
That mayor resigned due to corruption charges.
The toilet was stopped up and began to overflow with excrement. It's so dirty!
I will not forgive the bastards who disgrace my name!
Oh man, I got a ketchup stain on my T-shirt.
San-zui again, the 水 radical on the left.
The right part is 于, originally a pictograph of a rising object that hits something and then bends.
So 汚 is a pool of water that bends. It doesn't go anywhere, like a puddle, or a pond cut off from the sea.
Water that does not flow becomes stagnant, filthy, and stinky.
If you're in a restroom in Japan, and you're a lady, you should probably be prepared to see this sign in Japan:
汚物 here does not refer to excrement, of course, but rather pads, sanitary napkins, etc.