The phrase Iron Weathercock (also Iron Weathervane) is a derisive nickname given to the British politician Mary Elizabeth Truss (born 1975), Leader of the Conservative Party since Monday 5^th September 2022, and Prime Minister of the United Kingdom since Tuesday 6^th September 2022.

This phrase occurs, for example, in ‘The Iron Weathercock’: World reacts to Truss win, by Leo Sands, published on the BBC website on Monday 5^th September 2022.

The phrase Iron Weathercock (also Iron Weathervane) alludes to:
– Iron Lady (French dame de fer), a nickname given to the British Conservative politician Margaret Thatcher (1925-2013), Prime Minister of the United Kingdom from 1979 to 1990, with reference to Margaret Thatcher’s ruthless and inflexible stance;
– Liz Truss’s changing views on a variety of subjects—In particular: She was a Liberal-Democrat before joining the Conservative Party. She went from opponent of Brexit before the 2016 referendum to supporter afterwards. In September 1994, at the Liberal-Democrat conference, she made an impassioned speech backing a motion calling for the abolition of the monarchy, but, on Friday 9^th September 2022, the day after Queen Elizabeth II died, she hailed the late monarch as “the rock on which modern Britain was built”.
—For more details, cf. From nukes to corporation tax: Liz Truss’s history of flip-flopping, by Ben Quinn, published in The Guardian (London and Manchester, England) of Friday 14^th October 2022.

The phrase Iron Weathercock (also Iron Weathervane) translates French girouette de fer, which was apparently first used—and perhaps coined—by Laura Berny in Liz Truss : une girouette de fer, the title of a portrait of Liz Truss, published in Les Echos (Paris, France) of Sunday 10^th July 2022:

Liz Truss : une girouette de fer
Depuis ses débuts en politique en 2010, cette tory à l’énergie débordante aura servi David Cameron, Theresa May et Boris Johnson. Au prix de quelques revirements spectaculaires pour rester au pouvoir. […]
[…]
[…] Cette protégée de David Cameron était « Remain » avant de devenir une fervente partisane du « Leave » une fois le Brexit voté, puis […] elle a soutenu Theresa May dans l’âpre combat en faveur de l’accord de sortie de l’UE avant de rallier juste à temps son tombeur Boris Johnson.
Malgré ces changements de pied opportuns, cette fille d’enseignants de gauche, qui a rejoint les libéraux-démocrates avant de devenir députée tory en 2010, n’a pas varié depuis dans ses convictions de droite bien ancrées, voire libertariennes.
     translation:
Liz Truss: an iron weathercock
Since her debut in politics in 2010, this energetic Tory will have served David Cameron, Theresa May and Boris Johnson. Which involved some spectacular U-turns in order to stay in power. […]
[…]
[…] This protégée of David Cameron backed “Remain” before becoming a fervent supporter of “Leave” once Brexit had been voted for, then […] she backed Theresa May in the bitter struggle in favour of the treaty on exit from the EU before, just in time, going over to Boris Johnson.
Despite those opportunistic changes of positions, this daughter of left-wing teachers, who joined the Liberal-Democrats before becoming a Tory MP in 2010, has never varied since in her deeply held right-wing, even libertarian, convictions.