: having a very strong basis : very good and dependable. Financially, the company is (as) solid as a rock. Merriam-Webster (AS) SOLID AS A ROCK Definition & Meaning - Merriam-Webster
On a deeper level, being "solid" is a character trait. In an era of "ghosting" and flaky commitments, someone who is "solid as a roxxx" is a rare commodity. solid as a roxxx