/ˌrɑːk ˈsɑːlɪd/

rock solid

BrE /ˌrɒk ˈsɒlɪd/

NAmE /ˌrɑːk ˈsɑːlɪd/

that you can trust not to change or to disappear

The support for the party was rock solid.

extremely hard and not likely to break