My guess: When you're programming with money, you need a Solid programming language, run-time environment and compiler:
The execution of the program should be 100% deterministic. It should always run exactly the same way on all systems.
The language design should make it relatively easy to write good and secure code, and it should make it difficult to write bad and insecure code.
The compiler should give warnings about potential vulnerabilities in your program.
Smart contracts, once published, cannot be modified. They are solidly set in stone in the blockchain.
Nobody should be able to arbitrarily change a state variable of a contract without going through the procedures programmed into the contract.
The run-time environment and the compiler should be bug-free, tested, reviewed and audited.
I think these are all aspects of 'being solid' in a computer science sense: Solidity