Hi Vlad
The expression "to be wishy washy" refers to a person who can't make up his or her mind, to someone indecisive or to something uncertain.
For example, if you invite someone to your party and he/she first answers 'yes' and five minutes later 'no' and finally says 'I don't know yet', then you could say it's a wishy washy person.
Hope this helps!
Regards
Karen