The word 辈子 is formed by combining the character 辈, which carries the core meaning of "a lifetime" or "generation", with the character 子, a very common nominal suffix in Chinese that often serves to concrete an abstract concept or form a standalone noun. Together, they fuse into a single, fixed unit meaning "a lifetime" or "one's whole life", where 子 no longer carries its literal meaning of "child" but functions to complete and lexicalize the idea inherent in 辈.