overpromise and underdeliver, cast-iron (adjective)

He is asking fellow leaders to imagine how Europe will thrive in a dangerous world without a cast-iron American alliance. He deserves an answer

France's president may overpromise and underdeliver. But he is unfazed by those who accuse him of being pushy or difficult

ECONOMIST.COM

Europe faces an almost-existential moment, Emmanuel Macron says

He discusses the region's fragile place in a hostile world

cast-iron

adjective

UK 

 /ˌkɑːstˈaɪən/ US 

 /ˌkæstˈaɪrn/

 cast-iron guarantee, alibi, etc.

 

a guarantee, alibi, etc. that can be trusted completely

永不反悔的保證（無懈可擊的不在現場的證據等）

Can you give me a cast-iron guarantee that the work will be completed on time?你能向我確保按時完成那項工作嗎？

 

[ before noun ]

very strong

結實的，強壯的

He has a cast-iron stomach - he can eat anything.他的胃是鐵打的，甚麼都能吃。

(cast iron在劍橋英語-漢語（繁體）詞典的翻譯 © Cambridge University Press)

underdeliver

 To deliver less, or at an inferior level, than promised or expected quotation

参考】over-promise and under-deliver