Definition of rock solid

The phrase "rock solid" is an English idiom that describes something as being extremely strong, dependable, and stable, much like a rock that is unmovable and immovable. The expression "rock solid" can be traced back to the early 1900s, but it gained popularity in the 1950s and 60s during the era of rock and roll music. This time period saw a surge in the use of words associated with rocks, particularly in the context of music, such as "rock," "rolling," and "rocker." The phrase "rock solid" appears to have originated as a manufacturing term in the slang of the building industry. In construction, builders would refer to concrete that had hardened completely as "rock hard." Over time, this term evolved into "rock solid," which has since been adopted beyond the realm of construction and is now commonly used figuratively in everyday speech. The adjective "solid" originally came from the Old French word "soleel," meaning whole or complete, as in something that is perfectly formed and unsealed. "Rock" has its root in the Old English word "hreoc," which denotes hardness or rigidity. Together, these two words create a collocation that connotes immense strength and stability, embodying the very definition of unshakable substance.