From an email list, the definition of email list-washing:
Trying to identify the addresses in a spam list that will get you in trouble, e.g., spam traps and people with a history of complaining, so that you can mail to what's left. And also, by implication, dealing with complaints by saying "we took you off the list" without looking at the overall list quality.
The technical methods for removing stale bouncing addresses from a legitimate list are somewhat similar but the motivation is not.