弯弯 is the reduplicated form of the adjective 弯 (curved). Reduplication adds a more vivid, descriptive, or relaxed feel — often used in colloquial speech to describe something that is gently or noticeably curved, like a winding road, a crescent moon, or a smiling eye. It can function as an adjective or an adverb (e.g., 弯弯地). The pattern is common for single-character adjectives in spoken Chinese.