Solid bottom refers to any object or structure that has a sturdy base or foundation. To describe the same concept using synonyms, one could use terms like firm base, stable foundation, robust groundwork, secure footing, strong support, steady underpinning, unshakeable base. These synonyms convey the idea of something that is unmoveable and reliable. Other relevant alternatives include dependable base, unyielding foundation, durable backing, settled ground, and sound structure. Each of these synonyms emphasizes the importance of a sturdy and secure base that can withstand pressure and last for a long time.