The word 海誓山盟 is formed by combining the two-character phrases 海誓 (sea oath) and 山盟 (mountain pledge), where each phrase pairs a natural element, 海 (sea) and 山 (mountain), with a term for a solemn promise, 誓 (oath) and 盟 (pledge), to create a compound that uses the imagery of the sea's vastness and the mountain's firmness to emphasize the depth and steadfastness of the vows.