A Streetcar Named Desire, Scene 2

In context, Blanche means this as a compliment to Stanley.... a wishy-washy people would be considered weak.... spineless, without inner strength. Blanche is referring to Stanley's manliness.

"Well, I never cared for wishy-washy people. That was why, when you walked in here last night, I said to myself — "My sister has a married man!"— Of course that was all that I could tell about you."