The word "偷偷" is formed through the process of reduplication, where the single character 偷 (meaning "to steal" or "stealthily") is doubled. This repetition doesn't create a new meaning from separate parts, but instead intensifies and modifies the original concept. The doubling evokes a sense of repeated, continuous, or cautious action, transforming the idea of "stealth" into a manner of doing something furtively, secretly, or on the sly. The very structure of the duplicated characters visually and rhythmically mimics the hushed, iterative, and self-contained nature of an action done in secret.