copper-bottomed 
adjective British
Thoroughly reliable; certain not to fail: a copper-bottomed guarantee...
- Mr Brown failed to give a copper-bottomed assurance that Tony Blair would lead the party into the next election, saying only that it was ‘a matter for him’.
- He can keep quiet and hope Tony will repay loyalty on this occasion - but that has not proved a copper-bottomed guarantee on any issue in the past.
- He fuelled concern by stating that he could not offer copper-bottomed guarantees that there would not be further losses.
From earlier usage referring to the copper sheathing of the bottom of a ship
- flexography
- flex one's muscles
- flexor
- non-black
- nonblocking
- non-bonding
- non-book
- non-breakable
- nonc
- noncallable
- non-cancerous
- non-candidacy
- non-candidate
- non-capital
- non-Catholic
- nonce
- non-cellulosic
- non-central
- non-centrality
- non-certain
- non-certainty
- non-certifiable
- non-certificated
- non-certification
- non-certified