Question: What does it mean when something is "wishy-washy"?
Is it informal? Is it American English, British English or both?
Yes, it is informal and idiomatic. It is both BrE and AmE. Though there's no confusion in meaning, the dates of origin differ.
Wishy-washy means lacking in strength of character or purpose; ineffective; lacking in decisiveness; without strength or character; irresolute. Synonyms are namby-pamby, spineless, gutless and weak.
It could also mean thin and watery; pale, bland, wan, pallid, insipid. She always dresses in wishy-washy pastel colors.
It originates from washy (1560–70) meaning weak, watery, over-diluted: washy potatoes, or lacking in strength or vigor; insipid: a weak and washy production. Sometime early on, a reduplication occurred (as in flip-flop, or fuzzy-wuzzy), leading to wishy-washy.
Etymology Online gives its origins as 1690 (in the OED) for the meaning feeble or poor in quality (reduplication of washy) and first recorded 1873 in the sense of vacillating
For years people have been allowed to get away with this rather wishy washy idea that in the Sixties we smoked cannabis and it didn't matter. But in the Sixties it was a much less powerful drug, and now they have this stuff that is home-grown, which is at least 12 times more powerful." - former British Tory leader Iain Duncan Smith