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."
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.