1. To leave one's eating utensils in the kitchen sink for an extended period of time, for the purpose of cleansing.
2. Used in an euphemistic sense; i.e. to learn through an osmotic process.
Charles: Oh, don't you worry my dear, I'm sinkwashing them. As a matter of fact, the plates and serving apparatus at the bottom of the pile should be cleansed, they've been rinsing now since Saturday.
2. Chick (on phone): Manfred! How is Germany? Now that you've been there for two years, how are your language skills?
Manfred: Yeah, ok. I'm just sinkwashing my German.
You could shampoo your hair, wash your feet, or anything in between.
It's often, but not always, done secretly in desperation: there's no time to shower or bathe, or you don't have access to a shower or bath, like at work.
"You having a bad hair or what? You're gonna have to sink wash your hair before the meeting."