family silver
/ˌfam(ɪ)li ˈsɪlvə/ /ˌfam(ə)li ˈsɪlvə/noun
1Silverware belonging to a particular family, handed down from generation to generation, especially as an heirloom.
2 Figurative informal. Something considered to be of great value (materially or otherwise); a precious or venerable legacy. Now chiefly: specifically a well-established company, state-run industry, etc., regarded as an irreplaceable part of a nation's heritage. See to sell (off) the family silver.
Origin
Mid 19th century; earliest use found in Douglas Jerrold (1803–1857), playwright and journalist. From family + silver.