I heard it as "designed for" or "intended for", but with an implication that the lack of good design will not allow it to be used that way, so "suitable for".
I need a horse for jumping. How about that one?
That's a Shire. It's not for jumping, it's for pulling carts.
I want a rope for climbing mountains.
That one's not for climbing, it's for hanging washing on.
@Robert suggested that it meant "in support of" or "in favour of". Thatcher's statement was stronger, though; it meant that she was not designed to be turned, and therefore could not be.
All those in favour of turning Maggie Thatcher around, say "Aye".
There's no point; the lady's not for turning.
This was one of the things that led to her being called "The Iron Lady", the implication being that she was not malleable (with the additional pun that an Iron Maiden is an instrument of torture).