From Wikipedia:
"The lady's not for turning" was a phrase used by Margaret Thatcher, the English Prime Minister, in her speech to the Conservative Party Conference on 10 October 1980. The term has thus been applied as a name to the speech in its entirety. It is considered a defining speech in Thatcher's political development,becoming her motto.
The phrase made reference to Thatcher's refusal to perform a "U-turn" in response to opposition to her liberalization of the economy, which some commentators and Ted Heath had urged, mainly due to the fact that unemployment had risen to 2 million by the autumn of 1980 from 1.5 million the previous year and the economy was in recession, exceeding 3 million by the time the recession ended in 1982.
It was written by the playwright Sir Ronald Millar, who had been Thatcher's speech-writer since 1973, and was a pun on the 1948 play The Lady's Not for Burning by Christopher Fry, although Thatcher missed the reference herself.[6][7] Millar had intended the "you turn if you want to" line, which preceded it, to be the most popular, and it received an ovation itself, but it was "the lady's not for turning" that received the headlines.
The speech as a whole was very warmly received at the conference, and received a five-minute standing ovation.