A rock solid relationship refers to a strong, stable and dependable connection between two or more parties. It implies that the bond is unshakeable and can withstand any challenges or obstacles it may face. The term comes from the idea that something built on a solid foundation, such as a rock, will remain standing even in the face of harsh weather conditions or external forces. In other words, having a "rock solid relationship" means being able to trust and rely on each other completely, no matter what happens.