Right as rain.

[Graham-W]

Why should something be, "as right as rain"?
Not just alliteration surely?

[heathfield]

As good an explanation as any here

[swedeheart]

I've been wondering about that one...! The "as straight (down) as the rain" aspect seems very plausible to me. Cf. expressions like plumb in the middle

[fredpuli47]

'Right as ninepence" , referred to in the first link, was a very common saying years ago. Alone in the examples given it makes absolutely no sense at all ! It's a fine example of British irony.It meant, or came to mean, the opposite of what it appears to mean. There could be nothing 'right' about ninepence. Everything would be wrong about it. There was no ninepenny coin, though there was a threepenny one (a "thruppeny bit "'). It wasn't a proper fraction of a pound (20 shillings, 240 pence ). It was, in short, like 'a nine bob [nine shilling] note' an expression used for something which was seen as odd and non-existent (as of a homosexual: ' he's as queer as a nine bob note'). There was, of course, no such thing as a nine shilling note but there was a ten shilling one.