The word 天天 is formed through the full reduplication of the single character 天, which means "day" or "sky". This process of simply repeating the character creates a new conceptual unit that conveys a sense of daily repetition or continuity, transforming the idea of a single "day" into the enduring, cyclical notion of "every day" or "day after day". It is a direct and iconic morphological construction where the repetition itself delivers the core meaning of frequency and habitual occurrence.